RE: O.S. 55AX on Q500
Jeff....
the 55AX is just not designed to be an rpm engine. Would take some innovation and testing with prop selection to find speed on a quickie.
If you have one ...... use it... my guess is that with a 8.8x9.25 up front you might find the right engine load and power match.
If you are looking at buying an engine.....
The other engines mentioned are much better suited.
I tend to prefer the TT46pro and jett-stream combo as a good econo-speed setup. Even with the stock muffler, the TT46 with a 9x7 is good for 120mph+ in level flight on a Q-500 airframe. For most, it would be the fastest thing they have flown.
The OS46AX is described well by DaveN. That is a bit of a different animal, but with the right setup and prop selection it can indeed work well for speed.
Bob