I remember reading about this a while ago when i briefly had a BM2k, from what was said this mod was first done on the BM a while ago, way before all the impulse, angel and matrix ones.
But it wasn't popular as it didn't make any huge improvement as you can buy custom spring and LPR valve kits from numerous companies (Macdev springs to mind) which improve the stock one no end without defacing the gun.
Have a search around in on paintballcity or pbnation in the Bushmaster sections that where i read about it.. . . hope that helps
steve