6.04mm tightbores are plenty tight...6.03mm are good too.
It'll tighten up your groups a bit (CQBR barrels are still pretty short)...which will in turn increase your effective range (the range where you can reliably hit what you're aiming at).
Systema make solid barrels...I like the Black Python's for a good bang for your buck.
You might pick up 5-10fps...but a lot is dependant on your setup. It's not uncommon to simply take your rifle apart and put it back together and drop/pick-up a couple of FPS.
There's a couple of FPS comparison threads on this forum. I can never find it again for some reason. Drop of 30-40 fps sounds about right...but again it depends on your setup (compression, tightbore, hopup, etc...). But you're probably in that range +/-5 FPS.
Only way to know for sure is to chrony it.