Yes, we can get .20 and .25, but they're far less common than .177 and .22. Aside from the big-bore / custom pneumatics like quackenbush, etc., the most powerful 'standard' airguns come from Germany (Weihrauch, RWS/Diana, Beeman) and ironically, the UK (Webley, FX, etc.). Many many .177 springers and pneumatics are adverstised to be 900, 950, 1000, right in that range, so not much more than the limit on power by law there. A few are more powerful, advertised at 1,100, etc. My RWS 350, which is advertised at 1200 or 1250, is probably, near as I can tell, the most powerful .177 mass produced springer out there. I use extremely heavy pellets in it (Eun Jin 16 grs), to keep the velocities well under the speed of sound (around 1,050 -1,100 fps; I forget the exact figure of the SOS). Anything which goes from supersonic to subsonic before hitting the target impairs your accuracy, being the reason. I did in fact once chrono some lighter pellets in the RWS/Diana 350 in the upper 1,200s and even lower 1,300s, but I suspect that these are erroneous readings from my chrony, given the fact that it's more or less a POS chrony (Shooting Chrony), and that I've never heard of a manufacturer "under-advertising" their velocities. Plus I don't think I had enough light over the chrony. It appears that the Beeman Dual Magnum and Crow Magnum have the RWSs beat in power in .22 springers. The American-made Air Force Condor is advertised as "easily achieving 1250 fps" with .22 cal pellets,
which probably makes it the most powerful mass-produced .22, but it's a pneumatic, not a springer.
http://www.pyramidair.com/cgi-bin/show.pl?cmd_rifles=show_guns_manufacturer&Manufacturer=AirForce
As far as pistols, single stroke pneumatics, reservoir pneumatics, and CO2s are almost always in the 350 - 410 fps range. The Webley Hurricane is a little more at around 500 fps in .177. The RWS/Diana P5 will get 550, but I don't consider it an actual pistol because its so huge, it's more like a carbine without a buttstock. The Hurricane is probably the most powerful pistol-sized mass-produced air handgun:
http://www.pyramidair.com/cgi-bin/model.pl?model_id=7
http://www.pyramidair.com/cgi-bin/model.pl?model_id=482 (discontinued item, it says there, so get one if'n you're gonna get one).
So I don't think that the law there causes you to lose ANYTHING in air handguns, but just a smidge in rifles. Still, at 800 fps in .177, that will very easily kill birds up to crow sized, and with a good headshot, will easily kill rabbits & squirrels. A crow would probably fly a ways before dying however.