If it's not leaking stick with a stock valve. What little performance increase you
might get is not worth the extra money. If I was to buy a new valve I like the Rat 3.16, CCM Lower tube kit, and AKA-Lmp Lightning. I've heard some good things about the Madman Valve but have no personal experience.
I've found working out the springs so I have the IVG about 1/2 way through its travel and input pressure between 300-400 psi gives me the most consistant results. No one can tell you what color springs to use since there is too much variation in actual spring rates for a given color. You can set up a sniper to run at super low pressures but it gains you nothing and you loose effiencey plus I've found the get unstable shot to shot.
A good broken in regulator helps alot. I like Palmer's Stabilizer, AKA-Lmp Sidewinder, ANS Gen-II and stock WGP (though a pain to adjust).
Somewhere around here I did a right up about the steps to sweet spotting a sniper I'll see if I can look it up and link it here for reference.
Found my post on setting up a sniper
Sniper Setup