On my Parker I switched from an 80% let off to a 65% let off. This made the draw cycle a little smoother.
On my Darton, I just ordered the 65% let off. But I had to also buy the higher let off because it came standard.
But remember, my problem was in the shoulder.

As far as the grip, I would definetely experiment. If the low grip doesn't feel right to you, then you need to get a different grip. Check out loesch grips. You can custom order one in the exact position that you need. Also, shoot Arthur P an e-mail and ask your grip question to him. I know he hasn't been on the boards for a while and he may not return. I believe that he only shoots a high grip.