Mosin made a typo, he doesn't mean the 270% spring, he means the 170%. He's pretty spot on, but here are my suggestions, based on my experience building, upgrading and repairing 400fps AEGs.
Needed Parts:
PDI 170% Spring
Guarder Infinite Torque Up Gears
Systema or Deepfire Half-Tooth Piston
Madbull Black Python v2 Tightbore
Optional Parts:
Bearing Spring Guide
Modify Piston Head
Guarder Hard Type Bucking
Systema Torque-Up Motor
Large Handguard for G36
Large Type 9.6v Battery (a mini 9.6 wont give you much more over the 8.4 mini)
So essentially for about $130 or so, you can upgrade any AEG to shoot a nice, consistent 400 FPS, but you should look at spending $300 or so if you want to go the whole way with your G36. That is if you can do your own upgrade work. Otherwise you should send your gun out to a reputable person (like me

) and oay for them to upgrade it. You'd be looking at anywhere from $40 to $100 for labor, depending on how much the individual doing the work charges.