I was hoping that Shorty would post some stronger springs, although I was hoping for a little stronger... maybe 500-550 FPS. If it's worth doing it's worth overdoing!
High power spring upgrades work best in long barreled guns like M16A2's or G3's, which can benefit greatly from the increased accurate range, and coupled with tight bore inner barrels, pinpoint accuracy.
Strong springs are essential for snipers and marksmen, although it can be a hassle just to buy all the parts and get them installed, even if you can handle the pretty high price tag for motors and gears. I recommend a Systema Magnum Motor for producing high torque and a pretty darn good gear set.
This will make some cry. This will make all bleed. Play nice.