Maximum speed or climb comes when you are making the maximum power that the engine is capable. If you are only turning 9,000 static, it is doubtful that you are much over 11,000-12,000 in the air. The diameter and pitch are the two variables, so drop the diameter first by an inch and see where you are. I used to run 13-5's on a ST 75, and it turned a solid 12,000-12,500 depending on the day. It gave me unlimited vertical on a 7 lb. bi-plane, yet enough speed to hold knife edge from horizon to horizon.