Buy a M110 spring, fit it then chrono to work out starting fps. maybe run 1000 bb's / cycles through it, leave compressed in the aeg over night. then chrono. at this point you can either try taking a coil off or you can short-stroke 1 tooth.
another option check to see what if the spring guide and piston head have a thrust bearing installed, if one is missing you can perhaps install one with the M100 or remove one of them with a M110, that might make a small difference.
Either way 325 fps is perfectly respectable, sometimes it's better to leave as is
As @Rogerborg says Airsoft isn't an exact science, spring designs differs in how they are rated, add to that all the other variables involved etc.