On my QB build, no changes to the power plant, just plunked the original .177 top end back on. Oh, one change, I removed the 2 shims I had on the hammer spring. Accuracy was just as good as the .22, Kodiaks were in the high 800's, producing around 17 fpe, JSB Monsters were a bit slower but netted over 18 fpe. Funny thing, though, I just didn't like the feel of the .177 as well as I do the .22, so it's back to a .22 again, and whacking starlings with Barracuda Greens .
BTW, in case anyone missed my post on the main about the Greens, if you haven't tried these yet, and have a want/need for a non-lead pellet, do yourself a favor and get some. In my QB, they're doing about 930 fps for about 24 fpe, and are one hole accurate at 30 yards. Being light at 12+ gr., I suspect they'd also do well in CO2 only guns at lower velocity, but I haven't tested it yet, I need to give them a try in my XS 60. I've tried the Dynamic tin pellets before, had some luck with the .177 Sn-1, but never found a rifle that liked the .22's. Only down side is, I need to order more ! Later.