RE: O.S. 55AX on Q500
I've had quite a bit of experience with both the OS. .46 FX and OS. 46 AX, and without a doubt, the AX is a stronger motor on a quickie airframe. When I say stronger, I am not saying more RPM, as it does like bigger, lower pitched props. It is very happy with a sport APC 10x7 turning 14,200 rpm on the ground. The .46 FX liked higher REVS, like a 8 3/4" x 8.75 turning 16,500. Top Speed with either motor is very close to being equal, but the AX pulls through the turns without loosing nearly as much speed.
Not sure about the .55 AX, I've not heard a lot of good things about it, but it may just like a different prop than what others have tried, I would think a 10x8 might work well, as you won't have ground clearance for anything larger than 10".