I think the problem with having hp double the rated hp is that when you get into a situation that would bog down a tractor with the correct hp rating for the implement the implement is going to be the weak point and something is going to give on the tiller. My 660 is equiped with a slip clutch and if your using 2x hp the slip clutch might just slip when you get it in the dirt no matter how tight you adjust it and if your slip clutch isn't working correctly your putting your pto at risk of damage.
I guess you could just go real easy and till at very low ground speed and be okay. Just don't get carried away with going maximum tilling depth and get comfortable and think "wow, this thing really chews up the ground" Go easy and slow.
That implement pto rating is there for a reason.
Sincerely, Dirt