Sub 400 PSI was possible with a milled hammer, springs, and ported powertube and TPC. My cocker tinkered friend experimented with spring combos and he got to mid to high 390s. Pointless because it had no consistency or efficiency. Stick to the KISS theory and stick to the +500 output PSI.
The second valve spring is modified by a friend with medium stiffness. Third valve spring is much longer and very lighter, similar to Dargaan's valve spring setup and available in hardware stores per my request. My hammer spring is also longer but similar stiffness, I believe it is Nelson/cocker based.
I also have a 3 mm drilled and ported TPC.