I've gotten +6dB over ambient, and that's enough of a difference to be a genuine help. Not gonna mix the next Hamilton that way, but for community/school theater it can be useful.
I mixed a private school speech/drama conference (think: Thespian conference for private schools). It was 100% zone mixing with hanging mics and foot mics. The trick was to duck by -20dB the inputs not in use, so that the mic(s) nearest the dialog-producing actors could be turned up. The results were much better than the teachers/directors expected, many having tried area mics and had failed. I had several questions about what mics I used - but the real magic was between my ears and fingertips. Amazing what (was then) 30 years of mixing and professional sound reinforcement experience can do for a student show.