[Hectic]

then duster it is. Aside from doing a significant mod that would be hit or miss and is not reversable. Like cutting down the hamer spring or filing the valve striker down. Or drilling a tiny hole or two in the barrel to vent gas.
none of theese thinhs are accurate or reversable.
that or you can wait till there is a sniper clinic and pass the lvl3 then u can use yer 550 fps. I dont think they make low flow valves so duster is really yer only option.
you plan on gaming the rifle? Gas isnt all that consistant its not really a great option as a sniper rifle. At 400 you can "run n gun" with it. no rate of fire but itll be safe at 450 it wont really be any beter then it will be at 400.
so again best option is duster, or get lvl3 certification tho im not sure you can do both lvl2 and 3 in the same clinic so youll likely need to do 2 clinics.
that or buy a vsr or l96 spring bolt action. Swap some internals (cylender piston sears hop up unit and rubber maybe inner barrel) and grab a few springs one for 450 one for 500 one for 550 then as you getvyer certifications you can swap out springs or if fields arent big enough you can downgrade accordingly.