One thing not covered so far is your ground condition. It has a great impact on power requirements.
I just finished up on some "virgin" ground that hasn't been turned for many yrs. 2-3" depth put a big load on the tractor, my F24, I could see the temp gauge rise with the load and was moving in low/first gear. The motor was working quite hard.
Yrs ago I used a 4' tiller on my 50hp MF165 on a garden plot. I did dig deep and the load was a real strain on that tractor.
It's all about ground conditions.