My Bobcat 335 weighs about 9700 lbs and has 40.5 hp, non turbo motor. 19 gpm @3045 psi. That is about where my old 863 was when I started running a rotary in the late 1990's. It cut pretty good but isn't anywhere like what I run now. A 24 gpm machine @3000-3500 psi aux. flow would run a small head pretty good, I think.
A small rotary would take less gpm but would be more dangerous and the end product wouldn't be very nice. I think a fixed tooth, flail, or fixed knife head would work best. I think if someone wasn't looking for blazing speed but rather to use in a niche market, it would be a good tool. I'd personally like to try one but don't know if I could find enough work for a small mower right now.
"Life is tough, but it's tougher if you're stupid."