This is what I found from getting a used S6.
Rip the reg apart, check the thing shaped like a tophat, if the tip top part of the tophat looks chewed up like it got stabbed a lot with needle, get a rebuild kit for the reg. I had velocity issues just like yours and after a rebuild, It's back to normal. Didn't touch the internals on the gun itself.
The reg screw should be flush @ 300 PSI and you should be able to go from 200 - 300 using the IVG only.
EDIT: Nevermind. It's pretty much new...