The main use I have been thinking of needing a PTO motor for is a flail mower like you are doing. From what I have read, RPM seems to be more critical for flails than bush hogs, better to be high than low. I hate having to have full WOT (wide open throttle) output to make an attachment work properly. That is why I would even prefer a 7 cu in one in order to have a safety factor. And yes, it would be nice if the PTO unit could handle the full pressure. The total shaft length can also be an issue depending upon the setup. And I did intend to say "might be better". Running short on sleep these days.
Ken