You shouldn't have to go over 350psi. Too much pressure behind the valve works like a stopper keeping the valve closed.
Check the IVG in the lower tube, it shouldn't be more than three or four turns in from flush. Two turns is a good average.
I would set your inline to ~250 PSI and with your eblade in classic mode, hold down the trigger and up the LPR until the backblock just reaches the furthest back it can go, then add a quarter turn more for good measure.
Go to a chrono with your IVG set at two or three turns in and see what the FPS is at. Up your inline reg to raise the pressure, if it peaks and the FPS falls again, that's the point where the pressure in the valve is overcoming the main spring/hammer inertia.
The point right before the FPS drop is called the sweet spot, if your sweetspot is low, turn in your IVG 1/2 turn, lower the inline reg to ~220PSI and then find where the peak is.
You'll find that the peak FPS goes up as the IVG goes in. If you're needing more than four turns to get a good velocity, you should look into a spring kit.
Finding a heavier halfblock hammer wouldn't hurt either. Oh, and make sure you've got a fresh battery and that no wires are pinched between the frame and body.