There is a bell curve that effects the veloctiy of the pb's. For example, at 280 psi, you might be shooting 250 FPS, at 300 PSI you might go to 300 FPS, but at 400 PSI, youll be back down to 250 FPS.
this is because there is too much pressure and the valve cant stay open long enough, so upping the pressure is lowering the veloctiy.
Ill test my SR reg and see where its set at pressure wise. The best tool i have in my tool box is this item seen here. I added a gauge to an ASA and if i screw my reg into it, and then gas up the marker, ill see what pressure my regs set at.
edit: my SR's reg is set to 300 PSI