Before you get too carried away with changing springs and FPS, you should have a read of the site rules of where you intend to skirmish. Airsoft Billericay have a site limit of 350 FPS and here is a copy of the rules from their website....
Game Rules & Site.
Max power for automatic AEGs is 350 FPS ( all guns will be checked, there may be random checks whilst in game play )
Bolt Action Snipers 450fps
Snipers must carry back up weapons
Minimum engagement distance for Snipers is 30 metres
No Blind Firing, rebounds & gun hits don't count, foot shots under buildings do count.
MOST IMPORTANT - Take hits, if in doubt please class as a hit.
If people are not taking hits, don't shout & swear at them, tell a marshal only.
A marshal may randomly shoot you during a game with a pistol only, this is to help ensure people take hits.
Pyro's are confirmed on the day & must be inspected before use.
When throwing a pyro always shout "grenade" whilst throwing mk. 5 only.
Ground level sport so no digging holes or climbing trees.
Remove all rubbish from the site, if black bags are provided please use them.
Avoid head shots, although these are allowed it can be dangerous.
A lot of other sites set FPS limits to 328 but will tolerate up to a +5% variance.