Sounds like you have an air seal issue in your M4 more then a spring issue. I think the M100 should easily be pushing 365-380fps if everything is air tight. Might be as simple as changing the bucking out. Going up in spring rate just puts unwanted stress on your gb. IMHO
What Motor did you install when you changed out the stock one?
TM M14 base gun with Trigger Happy EBR kit. VFC PEQ15 and RS Aimpoint T1 w/ LaRue mount.
Only upgrades are a Prometheus MS100 spring, 6.03 inner barrel, soft bucking and a Guarder bearing spring guide.
Megalomaniac, I glad you got your new ACR. It sucks to be that guy, I feel your pain.
Let us know how it goes with the new stick.
At least they didn't tell you "duct tape". I tease.