My saito 100 is great. I run it on 10% cool power with an APC 15x6 prop turning at about 9500 RPM. Hover is at a little over half stick. I never, ever use full throttle when hovering, tailsliding, or recovering when falling out of a hover -- but I'm very, very confident in my recovery abilities. My model weighs 7 lb even.

I'm used to flying 35% aircraft with power to weight ratios nearing 3:1 and I think the Saito 100 is fine on this airframe.

You might consider an OS 91FX as a budget engine. I've seen it pull a 7.5lb UCD out of a hover with plenty of authority. But it has significantly less power than the saito 100, maybe no more than your saito 91.

Oh yeah, and the 100 is 0.1 cu. in. larger than the 91. That's nearly 10%.