The damage I can do to someone else is much lower if I hit them at 25mph on a bike than if I hit them at 25mph with a car. Yeah, I know, someone did recently die in just such a collision with a cyclist, but that is vanishingly rare.
Is it morally okay for bikes to go over a 20mph speed limit? If they're doing so sensibly, yes. Is it legally okay? Yes, unless there is a specific local bye-law.
Going 30mph through a 20 zone is unlikely to be possible very often, and its probably not a good idea very often either. Nudging above 20mph on a bicycle... I dunno, comparing that with a car, its like apples and oranges.