Personally? I think overkill...as in WAY overkill.

I try to always over-power my planes, but a pattern plane is not really meant to fly "hard and furious", but with finesse. A .91 would bore holes in the sky just great, but it would be a bit heavier, and not as precise on maneuvers. I would stick with a .70 or .72 4-stroke. The .91 would just be extra $$$'s that would be unnecessary, and probably not used to it's potential, as in, never at full throttle. The Aresti is a .40 size plane...possibly the BEST engine for it, is the .46 FX, or something along the line of a Rossi, etc.

If you are looking for something for the 91, I would be tempted to go with something like an Extra, .60 size. The Kyosho .60 Cap would be excellent, as well!

Someone is always willing to say "oh yeah, bigger is ALWAYS better", but that's really not true...

Just my $.02...