I had a .91 in mine and it was not underpowered. Was it fast, no, of course not. I could take off of thick grass in about 200 feet and climb out a pretty good angle, certainly never struggled. Loops and verticals of 2-300 feet were no problem. It does an awesome tail slide!
Do NOT use the byrons fuel system. It would not flow enough fuel to richen the OS 91 up.
I sold mind after a several flight since I did not like it. It did not fly all that well in my opinion. It was easy to fly, but it was not very fun to fly. I HATED the throttle-pitch coupling issues that mine had. Throttling up caused a significant pitch down for some reason, and coming off the throttle caused a pitch up. No other DF that I have flown has even done this.