I haven't tested this extensively, but from what I can tell, even if you don't meet the WPF requirements from your PT, you actually DO benefit from the extra PT. If you do not meet the WPF requirement, you cannot pick up ammo you've thrown, but you still do the extra damage from the PT above and beyond what you have the WPF for.
I've only tested it for PTs 4 - 6 with heavy throwing axes with a friend in the duel server. The only thing I varied was the PT, not the WPF nor the equipment of either two players, and I was at the WPF threshold required for 4 PT with the equipment I wore. I did enough trials of body shots that I can be reasonably confident that there was a significant difference in damage.
I could still be wrong, and it's possible the latest couple of patches fixed the issue. I'm sure it's not intended though.