What's important is the muzzle energy, not velocity. When we refer to 400 or 350 fps as 'field legal', that's assuming the use of .2g BBs. While using a heavier BB will reduce the velocity, the heavier weight of the BB will translate to the same energy. So a gun shooting 400 fps with .2 produces 1.49 Joules of energy. The same gun shooting a .28g BB will be shooting at 338 fps, and STILL producing 1.49 Joules. of energy.
The only way to reduce your velocity is to swap your spring.