Before Pesach, I was asked whether a convert who is a firstborn should fast on the eve of Pesach, in the fast of the firstborn. On the one hand, a convert is considered like a newborn child in terms of his familial relationships so his parents are not halakhically considered his parents. On the other hand, he still retains the "metzi'us" (reality) of being descended from his parents and therefore, for one example, inherits from them. While being firstborn is normally a halakhic status rather than a reality, in the case of the fast of the firstborn, it is possible that what is needed is only a reality and not a status. After all, the plague on which this fast is based applied to all firstborns, even animals, regardless of halakhic status. See Shulchan Arukh, Orach Chaim 570 and commentators.
So my thinking was that a firstborn convert should probably fast (or attend a siyum). I ran it by a prominent rabbi in Brooklyn and he agreed.
I subsequently found the following by R. Doniel Neustadt:
The status of a bechor born by cesarean section (16), or of a first-born non-Jew who converts (17), is a matter of disagreement among the poskim. It is therefore recommended that these bechorim participate in a seudas mitzvah and thereby satisfy all opinions (18).link
16 See Chok Yaakov 470:2; Kaf ha-Chayim 470:3.
17 Shevet ha-Levi 8:117.
18 Harav Y.S. Elyashiv (Seder ha-Aruch, vol. 3, pg. 44).