I don't think that a PD 10 is viable at all, so the limit on how much above your bow that PD boosts your damage already has a soft cap on it.
Please lift the hard cap on PD bonus above requirements.
This is a better solution to having the player base use more types of bows than buffing one slot bows stats, or nerfing the strong bow as was put forth in a different thread (stay away!)
This would basically affect the lower tier bows the most. Most upper tier bows, most people have PD 6-8 at most. They would have to go PD 9 to reach the current PD bonus cap, (meaning they can do it already, nothing changes there). Some crazy fool with no desire to actually hit anything with their uber bow might go PD with a Longbow, but honestly, it probably could use it!
What would be nice is a PD 8 Hunting bow to go after archers or xbow's or HA's with.
The class that would receive an indirect buff of sorts
might be HA's, but they aren't OP anyhow, and this wouldn't change much except they would run out of arrows faster