Dan
Your GT is 625 lbs and my CV is 650 lbs, your Merc should weight about what mine is.
My Merc red lines at 5250 rpm's, your will go 5500 rpm's, my gear ratio is differant also.
I have a 18 gal. tank under front deck, you have tank/tanks under splashwell (I think) ...
We should be pretty close in most of or spec's (give or take a few lbs).
All speeds are GPS ...
Alm. 19P Merc prop got me to about 48 mph & 5500 + rpm's
SS Lazer II 20P got me about 49 mph & 5500 rpm's
SS Lazer II 22P got me 52 mph & 5200/5300 rpm's (prop I'm useing now)
Dave's 21P Bronze cupped prop got me about 50 mph & 5300 rpm's ( would love to have one ...Smooth ... Had a very nice seat of the pants feel to it ... )
Other props in the 21p range got me in the high 40's with mixed results on boat handling & speed's.
Bet you will need something in the 21P range also ...
I alway carry a prop wrench in my boat so your welcome to try my Alm. 19P & 22 Lazer II at Red Wing.
I'll be there around 1:00 on Fri. if you get there early you could try some quick runs ...
Have you checked out the GT150 speed thread on the CGOA ?
My CV is a hand full over 50 mph, might be motor height ... but handles & ride great in the upper 40's.
The Prop Master will be there ....