As with everything opinion based.. it's all mutable... time is to. some people may actually like seat time.. and most people I know mow slower than I do.. I don't particularly mind a brisk mowing speed and a few bumps... The spring ride seat on my tractor is a real plus.
FWIW.. the dealer reccomended mower on that 32hp tractor I mowed with was a 6' mower. I opted to buy a 5', thinking I'd stay in the real power range... Luckilly I bought a few old fords to stick the 5' mower on, and instead used my stepdads 6' mower. The 32hp diesel spun the 5' like it wasn't back there.. i ran in 3/3 most of the time.. which was quite fast on that 12x4 tranny. Weeds 2-3' taller than the canopy went down with ease. About the only thing i ever mowed that made me drop into the '2' range was a full 3' tall thick lush grass.... Of course.. i still say the NH 1920 I'm refering to was way more tractor than ford touted it as... It would blow a tc30 away with one injector disconnected..
I never felt 'boxed' in or anything close to stretching the practical limits of that tractor in anything other than outright jungle undergrowth... and even then.. you could still mow it down faster than you could safely look for obstacles in front of you.. IE.. sometimes a lower 2 or (gasp)1 range was selected just to be able to find the hidden stumps and junk in the 3' tall grass..e tc.
In any case.. the hp rangees have been posted in this thread.. all the way from adequate to ideal, and the owner knows what to expect from a mower at the upper end of a tractor.. or having surplus tractor to spare.. etc.
I don't see a downside either way.. smaller tractor CAN get the job done, perhaps at a reduced speed... having a larger tractor gives you plenty of headroom , and opens doors for other larger tasks, but cost more, and more to maintain.
I own on both sides of that equation... my ford hundred series are not what i would term as 'awesome' at moving round hay bales,( adequate at best ) mostly due to reduced size and weight o f the tractors in relation to the bale size.. however.. they get it done... I'm actually surprised about the smaller 660 handling them...not so much about the 850... On the flip side.. I've had 9.5hp per foot of cut on a mower and knew I was wasting time with the overkill of hp, vs a larger mower, vs MY conditions.. and when it's all said and donw.. that's what counts.. the posters mowing habits and conditions.. not any of ours. What's average to many of us.. is likely not average to the original poster. What's average to a paid profesional mower is likely not what is average to the rest of us that mow our own land, vs letting it grow out of control.. then have to hire (FWJ) to come mow it for us.. thus what you see as average may well be inflated
soundguy