Concept claims only a couple fps loss from the 80% let off to the 99% let off option. I have never shot one, only read about them. I believe GV Holiday (may have his name wrong?) is a staff shooter for them. I am sure he could shed some light for your curiosity.

Shooting 70 pounds I like around 80% let off.
Shooting 60 pounds I like 65% let off better. Not enough holding weight for me with 80% at 60 pounds.