What velocity are you getting now, and with which slugs?
Do not discount the performance at say 850 FPS or so for accuracy until you actually try it. If you need big power for critter hunting, then by all means turn that thing up.
Try Ghosts advice, and you should be able to get .22 slugs sizzling along proper like.
My method will cost you around 100 bones max, and you will most likely top out around 900 to 925 depending on slug used. You may hit the high 900s with lighter .25 slugs like the Hybrids.