K&N claims gains of up to "50 horsepower."
However, I still haven't been given a plausible explanation of how an air filter, no matter how well designed, could possibly yield a 50hp gain over the stock filter. There has also been some discussion on the board about the K&N filter allowing more 'particles' into the engine.

"Stuff that restores horsepower/compression." You mean, like a ring job? ;-)