Call FX or send another email with your mailing address requesting that they mail you a new C3 o-ring and a new delrin washer as well based on the loss of FPS, under warranty.
Also, let's do some experiments now to find the loss in FPS:
First, does the power wheel adjustment have the adjustment screw that is backed out (so the slide moves back and forth with your finger even with the wheel on the "max" setting)? If the slide moves back and forth with the wheel on max then you need to tighten the screw with a 1.5mm hex wrench until the slop is removed. Do you have more speed now?
Next, well get the gun soapy in specific areas and shoot it in a safe target while you are standing away from it so you can see bubbles at the shot:
Air up the bottle nearly full, load 1 pellet into the chamber and remove the magazine, and put soapy water on the front valve knob, on the rear of the valve stem at the valve housing, on the pellet probe where it seals at the barrel. Now shoot it while watching the rifle closely. Did it "blow bubbles" from any of these places?