I had a Phantom down to 425-450 psi shooting 280 fps. The problem became valve popping. There was so little spring force and gas pressure in the valve that every single pump stroke opened the valve. I was thinking of installing a screw in the pump rod channel like on the Gargoyle to prevent over travel, but the safety mechanism gets in the way.
I abandoned the project and returned my Phantom to a regulated medium to high pressure setup. It shoots better anyways, and isn't finicky.