[yuhaoyang]

7mm G&P box is shit, every single thing in there either broke, or of such crap quality that they were unusable for me. Really, every single part except for the AR latch and cutoff. But shell is reinforced.
Buy a systema complete with magnum motor, the spring in mine shot maybe 420 out of the box.
Energy.... eeh, give or take it's still a V2 mechbox, and I can't recommend against it because I've never seen one. It's pretty dim-witted in theory though, it just has less inertia, and theoretically should provide a better trigger response and less over-cycling.
Systema's parts are all pretty much good-to-go for 400fps besides the piston, which has a high lemon rate apparently. Revolution is retarded, as nice as it seems, if you break one you're buggered.
Building your own; you can have a pretty easy time with it actually, something like a mix of modify, prometheus, and systema parts all work fine. (Torus shell, prometheus FTK, miscellaneous parts from modify or prometheus, and a systema motor SHOULD work perfectly). Be warned, this will cost around $500. But it's the same price as my full modify build... And that turned out well... *cough*