I have tested it and the supressor has little effect on the velocity. A touch less for begining the velocity measurement another foot away from the end of the muzzle, but nothing really significant.
And that, of course, doesn't address the leakage from the chamber. Too much pressure on the main spring in an effort to get the pressure (and thus the velocity) up and the chamber can't hold.
Geis
Oh, and by the way, when I included the link to the parts list, I didn't expect it to link within the message. If this bothered anyone, sorry.