don't lean it out trying to get more. you'll overheat and damage the engine eventually.
you can try going to an 11x10 (more pitch = more speed at a given rpm) but the reduction in prop diameter will give you less thrust. no big deal. 10,000-10,500 rpm is a good range for the saito 91. don't lean it to get into that rpm band. select the prop size to do that and keep the engine mixture optimized. you might be safe at 11,000. i'm not sure what red-line a saito 91 is at.
having said that that - there is no substitute for power.

go ys.