I'm jumping in kinda late here but I have to agree, there might be more going on than just your prop. By the way call OJ or Acme, give them your details and they'll tell you the right prop, eliminating a lot of quess work.

How many hours on the boat? How tight is the engine. We are talking a 10 yr old boat here. If it has been run hard and only minimal maintanence she could just be tired. I can run 1400 lbs of ballast and barely feel it. While my boat is much smaller, these motors have amazing torque.