One of the reasons the single pump air rifles like the Nitro Piston are popular is that they do not lose a lot of energy from the heat of compression.
The Pre-Charged Pnenmatics are not as efficient because they lose the heat. If you are not doing the pumping, it does not matter much.
There are some very powerful precharged pnuematics but they are so expensive that I consider them gimmicks. They will kill a buffalo tho.
My most powerful one is an RWS Diana model 48 in .177. It will shoot the standard pellets such as the RWS hobby or meisterkugeln at close to 1200 fps and that really is pretty strong.
I think the little Daisy model 880 which will pump up to around 850fps if you pump it a couple of pumps more than they recommend. They only cost $34 at Wal-Mart and that includes a scope. They can be extremely accurate. The biggest bargain in airguns to me.