I'd recommend going with the ics copper bushings. The bushing holes in the mechbox are a tad too small for guarder/systema etc to fit without alot of headache.
I did install guarder bushings in my ICS M4 RIS, however it involved the use of a c clamp, patience and a whole lotta cursing.
Also for your spring, have you considered the Prometheus MS100SP? I'm running one in the M4 right now and it's singing along at about 350ish...
Why do you need to be "clocking in" @ 380-400fps anyway?
Good luck with the tweaking. And props to you btw on your excellent grammar and posting format, it's nice to see new users n0t p05t1ng u531ng l33t h4x0r 5p34k and so forth...
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