My reason for still messing with my 91 is that it's more then enough for the airplane for me. I've hovered it low on every prop I've tried and it very confidently pulls out, it will even walk down in a tail slide and confidently stop where I want it to, so I can't ask more out of it. Plus I don't need a hunk of weight in the rear to get the CG where I want it, but to each his own. I'm glad you all are having fun with 120s in it, it seems to accomidate a wide range of engine selections, which makes it fun for all of us, that's the beauty of that huge wing and light weight.
CaptinJohn,
Mine has an OS on it, but my brother has a Saito 91 and the power seems to be very close, and the Saito is a lighter motor so you probably wouldn't need as much tail weight as we did if you want to move it back behind spec. As I've never seen one in person with the Saito that's my best guess, but "Mecam" posted a video some time ago of his UCD with a Saito 91 and it looked like it had plenty to pull out of a hover.

My personal opinion is the 91s have plenty of power to do the maneuvers, but it's not going to be TOC power to weight ratio. I bought my 91 specifically for this project, and if I had it to do all over again, I would. I really like the combination, and my background is 21 years in RC with many of those competing in pattern, so it's not like I'm just coasting around the sky and the 91 is just getting by. Everybody is different though, on this project I didn't expect ballistic missle vertical performance, just some close in, low speed, 3d. I got my fill of the balistic performance when I used to have my Saito 180 on my Goldberg Ultimate!