My CG Ultimate arf flys very well with an OS 91 surpass and a 14 x6 prop. It will do all pattern stunts and hang on the prop in a hover. It will not climb out of a hover. With enough entry speed it will climb out of site. I built it light with standard servos. Do I need more power- no. Do I want more power - of course, but I do not want the weight and only for 3d stuff. You will have a fast and nice flying sport bipe with your 91 fs that is as capable as the full size ultimate.

I will leave mine alone until I can free up my Webra speed 1.20 from my CG extra (wicked combo) or I just get bored (not there yet). If I install it I will need stronger servos, beef up the firewall and increase the diam of the aileron connecting rods. My only fear is it will be a different plane then. It is nice to fly something that can be graceful and scale like without making the knees wobble. This is the only one I have not overpowered!

BTW- balance will be right on the money per plans with the 91. If you go with a heavier motor the battery can be move to a shelf built behind the pilot canopy.