it all depends on your barrel length and bore. If you fit a carbine kit then the fps will increase, as would it if you fitted the DMR kit. If you want to set the fps reliably then you'd need an NPAS I believe.
When I had my G5 on the standard length it was always under 350fps even in summer. I converted it to HPA and played with different barrel lengths and bores along with the various nozzles and they can make a huge difference even with a fixed PSI from the bottle, I was actually able to drop my PSI requirement down to half what it was in my first set up by changing the nozzle for the hi-flow one so going the other way will make a difference but will massively depend on your set up.