My L has a 540/1000 PTO, both driving a 540 style PTO shaft (Yes it's a kubota oddity just like the three speed PTO's).
I can run my snowblower in light snow at half throttle in the 1000 PTO setting and get a good blow, but as soon as the snow get too thick it will bog the tractor right down.
It's all a matter of torque and available HP to the PTO.
After looking into this further, John deere is the only one that has tested it, so I'll have to use their info as a comparison.
Well I was a little surprised when I read the numbers, they are saying that on a tractor comparable to yours in HP specs you will still have enough available HP and torque for it to run the generator efficiently and safely.
Now for the big kicker they are saying that it would save about $20 a day in operational costs based on a normal working day, but that is with it also moving the tractor.
So lets just say for giggles that it's half of that to just to run the PTO, so $10, there are three work shift if you need it to run 24 hours so $30 in 24 hours.
In order to recoup your $800 investment it would only take 26.6 days of operation give or take, and doesn't even account for the savings in other maintenance and wear due to the lower RPM's.
So if you think your going to use it over the 26.6 days in the life of the tractor I would do it.