I was testing this exact thing today. I ran my bow from 65% let-off to 80% let-off. Made several shots through the chrono and I didn't see any speed gains whatsoever in either direction. All shots were within + or - 1 fps.

I did find some performance gains while adjusting cable tension in relation to dots/timing marks on forward side of cams (not draw stop peg marks on rear of cam). I believe there are 8 total dots/timing markson each cam. My bow was slightly past the 6th dot, meaning 6.5 dots showing past edge of limb. I advanced cams manually by pulling cables forward and installed a roll pin through roll pin hole in cam just below top limb and just above bottom limb.Roll pinrelieved tension from cables and allowed enough slack to remove cables one at a time for adjustment. Disconnected cables and twisted 4 twists clockwise top and 4 twists clockwise on bottom. This moved my timing marks to the 5th dot/timing mark and my bow picked up a consistant 4 fps. Still waiting on parts to install correct pivot spacers to bring bow up to factory specs.