SuperD, I have a newer two stroke 150 Johnson and it pushes a lot of weight. I am turning a 19 pitch that is double cupped, it will put me on plain really quick and it does really good on the top end. I run it at the elevations that you are asking about.
I live in Colorado and switched to a Solas 4 blade. 19 pitch I believe. Took forever to plane out with my 3 blade Merc prop but the Solas has been awesome at a good price. Lost about 2 miles at top end but still hit 45 mph which is plenty for me.
Dave, its a Venom prop and if i remember right it is 17 3/4" diameter? ( boat is at the lake)
The shop I got my prop from let me pay for one and they gave me six or seven to try, i would ask the shop that you deal with if they would be willing to do the same. This will help you get in the ball park and you can go from there.
I tried to have the same load in the boat that I would have on a normal day and I was at the lake that I spend most of the time at.