I am reading Kavvana: Directing the Heart in Jewish Prayer by Rabbi Dr. Avi (Seth) Kadish. There he makes the argument that the Anshei Kneset Hagedolah never established the exacting wording for the amida. A summary of much of this discussion can be found here. He seems to argue for the ability of anyone to adjust the wording of their amida. I would like to know if there is any contemporary (last 50 years) orthodox posek who supports this idea and paskens that a person can change the wording of his silent shemona esrei (as long as it doesn't violate the topic of the bracha)and still fulfill his obligation?
As far as I'm aware, there is no well-accepted posek who would permit this. Even R. Abadi, who has many unusual opinions and allows recitation of a shorter version of Birkas Hamazon, implies in that teshuvah that one cannot arbitrarily shorten the Shemoneh Esre. While additions to anyone's personal prayers are allowed (see Shulchan Aruch O.C. 119), this is only where an addition is needed, but not deletions or additions which are meant to be permanent (Aruch Hashulchan 119:2). This temporary allowance for additions, as far as I'm aware, is agreed upon by all contemporary poskim.
However, there are two people who might be considered contemporary poskim by some stretch, who did believe in such a practice. One is R. Joseph Heinemann, an Israeli scholar of the history of the siddur and similar matters, who wrote a book Iyunei Tefillah, where he discusses several themes about tefillah in general, including some halakhic concepts. Among them, he argues for the flexibility of shemoneh esre (and other prayers), at least in private.
Prof. Daniel Sperber is not exactly a mainstream orthodox posek (most of his opinions, especially regarding the ones involving women in prayer, have been attacked vehemently by others and are far from being widely accepted) but he did write a book about the development of the siddur, in which he argues for a more free-style prayer.
This opinion is based both on the history of the siddur, which he attempts to show has been rather flexible (and therefore there's no reason, he implies, why we should suddenly freeze its development when it's been changing constantly for centuries), and his reliance on various Rishonim who held this opinion. Among them the Ra"ah (Pekudas Halveiim to Berachos 11) who allows all forms of temporary (as in, not canon-changing) improvisations in prayer, and his student the Ritvah (Hilchos Berachos 6:14) who says the same thing. This also appears to be the opinion of the Rashba and Meiri (both to Berachos 11a) as well. Even though the Rambam (Teshuva 254) does write that any deviation from the original text is unacceptable, R. Prof. Sperber insists that we do not hold like him, because the Rambam specifically includes piyyutim as such deviations. Since Ashkenazic communities have been inserting piyyutim in their prayers for around a millennium, we clearly 'pasken' that the Shemoneh Esreh is more fluid and that one can actually change around the text of each beracha as long as one keeps to the theme and uses the same closing.
Prof. Sperber's opinion is elucidated in this shuir of his, and the article linked in the question is a continuation of articles than began by discussing his book on the subject: a critical review and a rebuttal.
While the following source is not a modern Posek, it still may be of interest. R' Yehuda Hachasid writes in Sefer Chasidim (ch.158) "When you pray, add your own needs to the formula of each Bracha according to its topic, because this increases your concentration. And if you can't add on to every Bracha because the congregation finishes earlier, add on to one or two Brachas, so that you shouldn't have to rush through the other Brachas."