Some of the other GT150 owners should post ..
I know Tim was getting some pretty good speed out of his GT150 / Merc 115 HP.
I have a CV16 that has same basic handling and performance as the GT150's.
I was getting about 50 mph/gps with motor down on transom and a 20P Laser II prop.
Rebuilt transom, lifted motor to one hole up from bottom with cavitation plate is about two inches above hull bottom, installed Baystar steering and have a 22P Laser II .. I'm getting about 53 to 54 MPH / GPS with half tank of fuel and good water.
My water pressure is still good at about 15 PSI at that speed. I haven't moved to bottom hole yet, but will soon.
Your biggest problem when lifting will be steering set up.
You'll have to cut a new hole to run cable or go with a Baystar.
I think there was a "GT150 performance" thread on CGOA forum ..
Search thru some of "Mark S" threads on CGOA.
Joe's GT150 thread is here …
http://forum.cgoamn.com/index.php?topic=4313.0