I don't think a .91 on a .60 size plane is overkill at all! Around here, that is pretty much standard practice.

On a downline, you DO have to use a little descretion and throttle management, so you don't rip the wings off, but that will be true of the .75 also.

As to the difference between the FX and the AX versions, the AX has an angled high speed needle valve, a new shape to the cylinder, and a new style muffler (which I don't like at all, but that is just personal preference...). The AX version also comes with a $50 higher price tag!