Hop-up and bb weight are huge factors. If you're chrono'ing at, say, 500FPS with .2g bbs, you'll be surprised how much accuracy you can gain by using really heavy bbs (.40g, .43g). I sure was using my Tanaka Model 29 (410 FPS) with .36g bbs as opposed to .25g bbs. The energy behind the bb will also be maintained for longer while in the air, making sure that whomever you hit is going to feel it. This, coupled with a good hop-up (I don't know what you kids are using these days...flat hop? R-hop? REAPS?) should be adequate. You can certainly do this under $200 if you're really, really savvy.
For 100% Tokyo Marui compatibility, you've got the JG BAR-10 and SOCOM Gear M700 as your cheapest options. If you want something cheaper, yet still upgradeable, there's the KART M700 which takes a mix of APS2 and Tokyo Marui parts. Either way, you'll want reinforced sears, a stronger spring, a stronger spring guide, whatever hop-up upgrades you want, and maybe some kind of part or modification that will ease the trigger pull. I'm a firm believer that tightbore barrels do not work, so don't buy one. Don't forget your .43g bbs, an optic, and extra mags and speedloaders.
EDIT: There is a modification you can do to your air nozzle to make the air flow better from the cylinder through it. It will require some milling, but the guy who did it got like a 40FPS increase without even needing to play with a new spring. Airsoft guns are NOT engineered very well from the manufacturer ;)
Yeah, I quit airsoft. No regrets.