20SF w/150 Evinrude Top-End

[rcnight]

Gents: Took the boat out to the lake yesterday and was hitting 40 mph at 5600 wot - one person w/less than half a tank. I'm running a 1996 150 Envinrude with a stock stainless 15x17 prop. Also have a T-top. How much of a difference will I see if I go up to a 15x19? How about engine mounting? How much foot should be below the hull? Does that translate into mounting holes? I have 3 holes. I'm now mounted in the top hole - or at it's deepest. Thanks for your input.