You could keep the same spring you have now and just get a short stroke spacer for you piston from tinbum.
i have a beeman R9, in a 177, shooting too fast for target, its shooting the 8.4 at 930, trying to get somewhere in the 12-13max in fpe, i contacted ARH and they only sell a kit for the 12-13 fpe i already have a kit in the gun, can any suggest a spring only, part number would be good and where to get one. chet