I think it will depend on what you're cutting, how tall and thick the grass is, and the terrain you're on, level, hilly, etc. For just one example: Bush Hog's web site shows the hp requirement for a 5' flat deck finish mower to be 15-35 hp. That's a pretty wide range. I used a 5' finish mower with a B2710 (about 20 PTO hp) and it never gave any indication at all of loading the engine. And I once talked to a dealer about 4' vs. 5' mowers behind a B7100 (13 PTO hp?) and he recommended the 4' one, but said he had some customers who were happy with 5' ones but that they were pretty heavy for the 3-point hitch to pick up without front end weights added.
I just bought a TC18 and the PTO HP is around 14. I have used the RFM only once since I've been using the MMM. But the TC18 handles my 60" Caroni RFM very well.
Kubota L3560, Toro 52" and 60" ZTRs, Kubota RTV 900
As Bird mentioned about the B7100, my brother had a MF 1010 (13 PTO hp), and he was able to use a Landpride 5' RFM. Tall grass woulld require you to shift into a lower gear, and you had to have 150 lbs of weight on the front, or you could wheelie the tractor /forums/images/graemlins/blush.gif His B2400 handles the mower with no problem.
Will