If you want to get the power down on a GBBR you are best to install an NPAS which will give you adjustability for different sites. I have one in all mine , and can hapily get my L119 which is based on a WE M4 down to 1.1 joules (just under 350 fps on .20's although our local site chrono's on power and I run .30's) on red gas or lower on green gas.
You will need to check the fps / power limits for whichever CQB site you are wanting to play at and set the NPAS up from there.