Are there any accepted authorities of Halacha that permit a man to eat (i.e., "real" eating such as consuming cake or cereal) before davening Shacharis under "normal circumstances" (i.e., a healthy person with no other reason to eat then other than "they are hungry")?
It says in Hayom Yom for Yud Shvat that when Rebbetzin Rivka (who was the wife of the Rebbe Maharash, the fourth Lubavitcher Rebbe) was eighteenth she felt ill. The doctors told her that she must eat breakfast. She didn't want to eat before Davening so she started Davening early and afterwards eating breakfast [I assume that means that by her normal wake-up time she was already finished davening and could eat].
Her father-in-law (The Tzemach Tzedek of Lubavitch) said "A Jew must be healthy and strong..." He then said "You should not be without food. It's better to eat for the sake of davening than to daven for the sake of eating".
In general, in chabad one learns chassidus before davening which results in shacharis starting 9:00-10:00. Therefore, the custom in Chabad is to eat before davening and in most (if not all) chabad yeshivas, mzonos is commonly served before davening.
This reminds me of a Rabbi that I knew who used to say "Better to eat and think about the Davening, than to Daven and think about eating".
The Rivvivos Efraim says there is no Heter.
However, HaRav Ben Tzion Abba Shaul in the notes here mentions based on Orach Chaim 89:4 that if you are hungry or thirsty and can not have Kavana properly then you may eat before Davening.
Shulchan Aruch 89:4: "One who is thirsty and one who is hungry - behold they are in the category of the ill. If one has the ability to focus one's mind, one should pray. If not, if one wishes, one should not pray until eating or drinking."
Rambam's Mishneh Torah, Prayer 5:2: "...if he can concentrate, he recites the prayer; if not, he should not pray until he has eaten and drunk."
Aruch Hashulchan 89:24: "...even if he doesn't have an ailment at all, rather [he eats] in order to strengthen himself...even real kinds of food and drink, it is allowed."
From the above it appears that if one is very hungry he must eat before praying according to the Rambam. Beit Yosef 89:7 takes it down a notch and says that eating is optional and subject to one's discretion - "הרשות בידן." Mishnah Berurah 89:26 agrees with this conclusion even in light of the fact that today we are not able to concentrate during prayer even when not hungry.
This article by Rabbi Yosef Melamed of Denver Kollel answers your question:
The Shulchan Aruch (Orach Chaim 89:54) rules that someone who is hungry or thirsty may eat or drink before davening.
The Mishna Berura qualifies this ruling as referring to one who is very hungry or very thirsty (Mishna Berura ibid.:25). The source of this halacha is the Rambam (Hilchos Tefila 5:2), who writes that in this case, one is obligated to eat or drink before davening in order to facilitate proper concentration for davening. However, the Shulchan Aruch only writes that someone who is hungry is allowed to eat, which implies that one is not obligated to do so (Magen Avraham ibid.:13). This is because nowadays there is a general lack of concentration when people daven, and it is assumed that one will not have the proper level of concentration even if he eats before davening. However, one still may choose to eat if his intention is to better his concentration (Magen Avraham ibid. based on Bais Yosef 89) .
The Pri Megadim (Orach Chaim 89 Eishel Avraham 13) explains that the reason for this heter (allowance) is that one is eating for the purpose of serving Hashem better, not to gratify his own desires, and so this eating is not regarded as haughtiness. According to this view, the prohibition does not apply at all.
According to the opinion of the Pri Megadim, one would be permitted to eat and daven later with the minyan. The eating is intended to enable davening with proper concentration and with a minyan, and, according to the Pri Megadim, this removes the issue of haughtiness altogether (Shu”t Mahari Shteif 41). One who wishes to follow the lenient opinion and eat before davening in order to concentrate properly while davening with a minyan should preferably recite the first chapter of Shema, as well as a minimum prayer containing a praise, request, and thanks to Hashem , as this fulfills the minimum obligation of prayer and thus suffices for the purpose of relying on this leniency (see Shu”t Mahari Shteif ibid., Shemiras Shabbos Kihilchasa 52:fn. 48).