It can't be his max potential. That's like saying Cell's max potential is his FP PC/SPC power levels because he declared himself perfect at the time. If Vegeta started out weaker than Zarbon and still wasn't maxed out by the Boo arc then Cell or Boo maxing out so early makes zero sense.
Tough to say. We have a precedent for potential unlocked people getting rapid increases similar to Ultimate Gohan in Super with Kuririn and Gohan getting random pseudo-zenkais from tough battles so it's possible Ultimate Gohan could've gotten stronger if he kept training, and Goku and the others' limits would also be retconned later. After all Vegeta acted like he hit his base form's limit in like the Freeza or early Cell arc (saying he was seemingly at his limit post-Dende or during gravity training pre-SSJ), only to massively power up afterwards. Toriyama also seemed to think Vegeta's limits were above Ultimate Gohan's in BoG while stating he chose that post-Boo, pre-EoZ time period because that was when everyone was at their physical prime, and that was after he reread his own manga and hadn't yet retconned the shit out of Z's logic with future Super arcs.
