As a general rule in simple terms:
Reccommended = "more & it becomes a handful to drive"
Allowed = "more & you break it"
Insuance wise it's likely the Max, although if you do a search on here there seem to be quite a few companies don't seem to care when you ask for a quote - but I'm sure would suddenly care if you hurt someone / broke someone else's boat. I'd wait for the US crew to fill in the details as I'm sure your insurance will work differently to over here.
As for stress on the transom, yes, more HP is going to give it more abuse to withstand, and as a general rule, more HP = More weight. You may find around that HP that the extra weight of a 30 that's a de- tuned 40 will make the handling worse, so you're actually better with a 25 that you can open the throttle on. Could you borrow an engine to test with?
Also bigger engine on a tiller = sore arms after a while.