i get nearly that speed with a 18' tahiti bubble deck. (actually a "runabout") and it still has the harmon marine 390 HP 455 olds, single 750 vac holley on a torker. no high dollar stuff here.
what pump are you running? mine is a berk JC. i would look more into the pump for added speed. no matter how much HP you put into a non efficient pump, you wont gain any speed. it will only work as good as it is. a properly set up pump can give you an amazing gain.
as for the hull, what i have heard is that most Tahitis were more of the river/lake boat. a bit heavier glass but stronger. they made a bunch of em.