Great post Tys. I agree 100%, get to know your gun before you make any changes to it, and then adapt to how you play.
If you like to be in close, aggressive style with fast reaction and target acquisition, build a compact gun designed for tighter corners, lighter for fast movement, and better control. You dont need high FPS for this, just precision and ROF.
If you prefer to hang back, pick your shots, support the guys I mentioned above, build a slightly longer gun with some upgrades to your compression chamber, a good tightbore barrel, and a good hopup unit. It does not have to be huge, M14 length or what not unless you like that.
Find out how you like to play, and tailor both your AEG and your gear to that.