Fastest ever? I think it was NASA that used an airgun and pellet to simulate meteor impacts to observe the ejecta pattern and what happens to the meteor. I calling on memory from a long time ago, but I think it was over 20,000fps. Nice... I know that's not what you meant, just thought it was interesting.
As for realistic store bought stuff you're probably looking at closer to 1500fps as a max using useless super light pellets. My Benjamin Titan for example can hit 1290 with a 5.2gr pellet, corrected to sea level it would break 1300, and that's not even a magnum, just a standard $90 gun.
For realistic usage everyone stays under the speed of sound. Most guns are stuck there anyway, but if you break it you lose accuracy and noise goes way up so there's no point. Super accurate air guns usually stick to 500-550fps, guns for hunting are usually more accurate at 900fps or less, but it varies on the gun. No point in having a magnum hunting gun and tuning down to 550 for accuracy, but if an 18gr pellet shoots at 850 and a 14gr shoots at 950, chances are the 18 will be better all around for a variety of reasons including the weight and BC.
So I guess it all depends on what you mean by practical. I have several that shoot .177 at 1000 to 1050 and they're fine for hunting, but using a heavier pellet to bring then down to 850 is probably better for most applications as long as the gun likes the pellet. Airguns, especially springers can be finicky so what works on most might not work on your gun so not just velocity and weight, but the make/model of pellet is a factor too. The best thing is to try as many different pellets as you can to see what works for you and yours. I even saw a website once that has a pellet exchange program where people exchange small numbers of pellets rather than waste $10-20 per tin. I usually stick to JSB and H&N pellets, but RWS and Crosman Premier are also good. If your gun can't shoot any of those then chances are there's a problem that needs to be fixed before you can continue.
So basically ~550 for max accuracy, ~900 for max hunting accuracy, and supersonic for max period unless you're just goofing off.
Hope this helps