They are all pretty similar: 350-420 fps with typical 7 grain pellets. That holds true for the HW40, Gamos, Beeman P17, FAS, Webley Nemesis.
The IZH-61M and the HW75 are at the top end: 410-460. Because they are bigger and heavier, and compress a bit more air than the smaller ones.
SSPs are pretty simple things, and there's no real way of tuning or "modding" them for much greater power apart from fitting a longer barrel.