Not sure what your actual goal is, but trying to get any kind of accuracy out of a spring powered pistol is a real challenge. At least I'm not up to it. Single strokers or multi-pumps are in a different league entirely, due to the essentially recoilless behavior. That said, there are a couple higher powered spring pistols out there that will make over 500 fps in .177, the HW45 and Diana LP8, if you want to give it a try. Enough for sparrows, but not enough for squirrels in my opinion.
If you are looking for a compact springer for close range pesting, start with something like the HW30s. Shorten the barrel, shorten the stock, and put a red dot on it. More accurate and more powerful than any springer pistol can be.