You said it seems to give the engine more power, which means it suits the engine/airframe combination. That says it all. If the engine were overheating you would notice it. You might try an 11x4 or the suggested 10.5x4.5 and see how it does. Props are a pretty cheap part of the hobby and it is worth experimenting, as you are finding out.

I'm not surprised your prop worked well, because the LT-40 is a big draggy air frame, so high pitch props are apt to be wasted on that plane.

Also, I agree with others who point out that this engine does fine on largish props. I had an OS 40 FP that seems to do best on an 11x5. But that was a small plane.

Jim