I'm running into a similar problem. My Power Plenum is a first run gun that should have the 57mm spring. Anything over 145 bar and I lose velocity. I was pushing .25 FX Hybrids at 1060 with the pellet liner with very tight extreme spread, but can't get them over 1040 with the Superior liner and the ES is all over the place. I'm looking into dual transfer port, lighter return spring, and a pin probe.