Don't worry so much about HP numbers. The GPR hull is one of the best and most versitle hulls ever made. It can be settup for top speed runs or CC racing and anything in between in a matter of minutes by just adjusting ride plate and trim tab angles with washers between the plate and hull. It has been proven the GPR hull is one of the fastest and thats why many of the speed record holders use it and normally put sea-doo 4-tec motors in them and run 90+mph. There's even 1 guy who has converted his GP1300R into a surface drive propski and has run 113mph with that settup.
A friend of mine has a '00 GP1200r with Speedwerks tripple pipes, Speedwerks ported cylinders, Speedwerks 150psi head, and 48mm Novi carbs. His ski runs 78mph on gps with more of a handling settup than I have. We have lined up many times and we are dead even out of the hole up to about 65-70 then he starts pulling very slightly away from me. My ski is settup a little looser to get more speed but still hanldles very well. I'm running 75mph gps on my stock 1200 lower end, the cases have been ported, the Carbs were modded by Island Racing and the choke plates and oil injection were removed, stock oem non-ported 1300 cylinders with and oem 1300 head milled by Jim's performance, VF3's, and a stock single pipe exhaust. There's alot of little things done to mine as well that all add up.
I have raced many stock RXP 215hp ski's and beat them out of the hole and on top end. I've also raced the new RXP-X 255hp ski's and we're about even all the way from bottom to top. My ski probably has about 180-200HP The sea-doo's hull and pump are not as efficient as the GPR.