Okay quick fix for you.
I take it you still have the standard velocity adjuster and not an after market part. Its on the back and is a round screw with Knurling around it and a little black Grub screw in it.
Right take an allen key and remove this grub screw and put it somewhere safe. Now fire of the marker and test its velocity (not at someone as you should be fireing over 340 fps and at 325fps you can break finger bones
)
Does it appear to be firing further??
If yes and the chrono reading is above the 300 fps limit you need to put the grub back in and reset it to be below 300fps.( 280 fps is the best setting as this allows for any spikes)
If the answer is no I would try a harder spring and try to set it up with that.