G&G makes a great GB. Good meterials and already had the charp corners of the font of the cylinder housing rounded out to prevent cracking. Also G&G boxes will drop right into about anything. Realisticly though, as long as you're not using anything over an M120 spring, pretty much any gearbox will do.
If you really want to focus on ROF you're deffinitely going to want bearings and you'll want them wherever you can put them. Replace your bushings with bearing bushings, ball bearing spring guide, ball bearing piston head all that good stuff. Speedy ROF focused rigs (~30rps+) are known to be less reliable, so you want to cut out all the possible fail points. Run a light piston with few metal teeth. Seal up the air seal components and run a weaker spring. Make sure your angle of engagement is good to keep the piston from being torn up quickly. I have a Gen1 (the best ones) KWA M4 CQB that I've had set up for a speedy SoGo gun for over 2 years now with no failures. I'm running an AIM Top Infinity Speed gearset, factory KWA polycarb piston, poly carb ported piston head, factory KWA poly carb cylinder head, AIM top air seal air nozzle, a Systema M85 spring, and a Matrix Turbo 3000 motor. I consistantly get about 280fps with a .20g out of it at around 20rps+ with a small 9.6v battery and over 30rps with an 11.1v lipo. The lipo will actually casue the dreaded 'pre engagement' so I don't recomend it unless you hand your gun off to some one who knows how to 'short stroke' a gun.
