The rifles in standard Match format are already sub 12ftlbs and running at around half of that power. I presume that you want it to be closer to the 12ftlb mark (which I believe is difficult to achieve). On that basis here's a copy of my contact with FWB regarding similar.
Q : can the Muzzle velocity of the Rifle be increased at all, even if by a small amount. If so, is this something that the owner can carry out, or are the services of your UK representative required.
A : the muzzle velocity can slightly be increased. For doing this the action will have to be taken off the stock. If the rifle is in shooting direction you will find in the rear section of the trigger housing the firing pin screw beneath the absorber tube (see attachment). By means of this screw "B" the muzzle velocity can be adapted a little. Screw "A" secures against independent twisting of the firing pin screw. Loosen screw "A". Set the velocity - turning the firing pin screw clockwise will increase the muzzle velocity - turning it counterclockwise will decrease the muzzle velocity. Please note: if the firing pin screw is screwed in too much the firing pin will open the valve and the compressed air cylinder will deflate air. As soon as the muzzle velocity is set tighten screw "A" again.
As far as I am aware there are a couple of specialists out there who can modify the 800 series for 12ftlbs but TBMK they need to change a few parts to do so. I believe that FWB themselves had problems doing it on their own rifles in the factory (for an extended period of time as far as I am aware).
HTH, Vic Thompson.