Daystate Air Wolf MCT .22 adjustments

I tried the heavy JSB 25.4 redesigned in .22 caliber. I shot a three shot group at 65 yards and they nearly touched. This gun is completely stock and very accurate. I have chronied it with the JSB 18.3 pellets at 935 FPS. I shot a three shot group at 65 yards using this pellet and it was very slightly larger than the 25.4 grainers, but 1/2 the drop of the heavier pellets. I would like to use the heavier pellets but I chronied both and the heavier 25.4 pellets were just over 800 FPS, hard to believe there is that much difference in the velocity. I would like to know if there is a way (without sending the gun back) of adjusting the velocity to around 900 fps. I believe this would make a squirrel hammer out of this gun.