ORIGINAL: c550
Doesn't os make an .80 four stroke now? The only real issue with a .91 is the fuel tank if your not worried about the cutouts in the cowl. I don't remember if there is more room in the nacels for larger fuel tanks, but if you are ok with a slightly shorter flight I think that would be the only real change.
Yes, the new OS .81 alpha's are a bit pricey but they will turn the same prop as the old .91 Surpass motors and are much lighter and more compact.