G-Spec.
You wouldn't want to go for just 400, you would want more up near 500. A bolt action at 400 is like, the slowest semi-auto AEG ever. The hop will be more accurate but you're really just limiting yourself.
Well anyway, with an upgraded spring you would then want a new cylinder, cylinder head, piston, piston head. Maybe a reinforced bolt handle. A tightbore would be much more important than all that though. This IS supposed to be a sniper rifle after all.