Was there ever a study done on the percentage of Reform Jews who would be considered Jewish according to Halacha?
No, and it would be nearly impossible to determine. Every modern survey and census of Jews in America has been performed with the widest possible definition of Jew, in order to obtain the fullest and least-controversial numbers. This usually translates to counting someone as a Jew if they identify themselves as Jewish. (Source)
For example, a recent census, the National Jewish Population Survey 2000-01, primarily counted someone as a Reform Jew if they considered themselves to be one. The more stringent definition was simply those who are members or are affiliated with to a Reform synagogue. (Source) Neither definition gets anywhere near "Halakhic."
The most recent census (as of this edit), the Pew Research - Portrait of Jewish Americans, also used self-identification as their primary criteria.
Even if one were to attempt to find out, they'd run into the following problems:
Reform Judaism accepted Patrilineal Descent in 1983. Any survey or census performed since will have been done according to their standards, which means that the latest generations of Reform Jews will only needed to have just one Jewish parent, and the data will not distinguish which parent it is.
"If the child of a Christian father and a Jewish mother is not raised Jewish, the child is a Jew according to the Orthodox movement, but not according to the Reform movement. The matter becomes even more complicated, because the status of that interfaith child's children also comes into question." (Source)
"Reform Judaism stresses the importance of being raised Jewish; if a child is born to Jewish parents and was not raised Jewish then the child is not considered Jewish." (Source)
The most recent study, performed by Pew Research, underscores this issue:
"The number of Americans with direct Jewish ancestry or upbringing who consider themselves Jewish, yet describe themselves as atheist, agnostic or having no particular religion, appears to be rising... and most U.S. Jews (62%) say being Jewish is mainly a matter of ancestry and culture, while just 15% say it is mainly a matter of religion."
Thus, there are Jews who would be considered Jewish by Halacha, but not by Reform!
In addition, the Pew Survey illustrates yet another issue:
"Even among Jews by religion, more than half (55%) say being Jewish is mainly a matter of ancestry and culture, and two-thirds say it is not necessary to believe in God to be Jewish."
I am far from an expert on this issue, but if I recall correctly, there are many problems with one's halakhic status if one denies God.
Additionally, Judaism counts the children of converts as Jews. However, Reform conversions are not performed to traditional halakhic standards, rending their children non-Jewish according to Halakha.
Finally, even if the mother was halakhically Jewish, there are many problems with potential mamzeirut, as Reform Judaism does not conduct halakhically valid marriages and divorces. While Mamzeirim are Jewish, they are many halakhic issues regarding their integration into the broader Jewish community, and thus any census of "Halakhic Jews" would have to include them as a separate category.
Non-halachic Reform Jews are those whose mother was not Jewish, yet they consider themselves Jewish because of either a heterodox conversion or patrilineal descent. We can mostly figure out the demographics from particular questions asked in certain surveys.
According to the 2013 Pew Study on American Jews, 10% of American adults who identify their religion as Judaism and their denomination as Reform say that their mother was not Jewish. (19% if you include those who consider themselves Jewish "aside from religion" and Reform, but we're going to focus on those who consider themselves associated with Judaism in some form.) Assuming that the percent of these people who converted Orthodox and later changed to Reform is negligible (which seems to be backed up by the data), we can assume that practically all of these individuals are not halachically Jewish.
This gets more complicated when we consider the descendants of the female portion of that group. Any of the children who still consider themselves Reform Jews would say that they do have a Jewish mother, and the same for their descendants on the female line. How many people are there in this group?
At the time of the survey, Reform Jewish women who say that their mother was not Jewish were 7.8% of the American (adult) Reform Jewish population. (Yes, the gender balance is massively skewed.) We have a fairly small sample size to work with for this group (less than 60 people), but we can get estimates of some basic points: 85% of the portion of the group with children have Jewish spouses or partners according to the respondent (38% are married to Jewish spouses overall), they have 0.8 children on average, ~1/3 of which are currently minors living in the respondents' homes, most of which are being "raised Jewish". About 55% of people raised Reform Jewish (regardless of whether they say their mother was Jewish) continue to identify as such as adults. Pew did not ask about the current religious affiliation of the respondents' adult children who don't live with their parents, so we'll have to rely on some educated guesses based off of what we know of Reform Jews in general.
So, how many adult Reform Jews have mothers who self-identify as Reform Jewish but are not halachically Jewish? Without taking into account the considerable denominational switching between groups, the amount of Reform converts who were raised not Jewish by non-halachic Reform Jewish mothers but later started to identify with Judaism (Pew only asked those with no Jewish parents whether they had converted), or the differences in norms between generations, or the fact that many of the relevant mothers might be no longer alive, the group might be about 2-3% of American Reform Jewry. There may also be a smaller group of third-generation descendants of these groups, but probably not enough to make a difference.
So, approximately 87% of adult Reform Jews in the United States are halachically Jewish.
In Israel, according to the 2015 Pew report on the country, 3% of the Jewish population identifies as Reform. According to the report, 100% of the 89 Reform respondents said that their mother was Jewish.
While, no exact number can be known, estimates are possible.
In a survey done with Reconstructionist, Conservative, and Reform rabbis, no Conservative rabbis would perform an intermarriage, 36% of Reform rabbis would, and 62% of Reconstructionist rabbis would.
In a population survey, 33% of American Jewish Families were "interfaith families", while another study says that currently, just over 50% of marriages are "interfaith marriages"
According to this article, It's hard for me to tell if 33% of intermarried Jewish families had a Jewish mother, or if 29% of Jewish woman, and 33% of Jewish men were intermarrying. The quote is..
Between 1980 and 2004, women, for the first time, were intermarrying in similar numbers to men (33% to 29%, according to the 2000-2001 National Jewish Population Survey).
Based on this information, About 18% of Reform Jews today would not be halachically Jewish. And 33% of Reform children born now, would not be halachically Jewish.
While not specific to Reform Jews, the National Jewish Population Survey from 2000-01 (which was just enough time after Patrilineal Descent for college kids to have been born) the found that only 48% of Jews in college had two Jewish parents. Which would mean that 26% of Jews in college have only a Jewish father, and thus aren't halachically Jewish (and this is even assuming that all self-identified Jewish parents are halachically Jewish). Now if that was the case 13 years ago and intermarriage rates have increased or at least stayed the same, and the problem compounds by being left unchecked, then the current and future generations of Jews are going to have even higher percentages of non-Jews in their midst.