No, it doesn't hurt your horsepower, you might actually will gain a couple actually using the k&n exteme tops. Hot Rod did an article back in 2004 when they were comparing different air filters on a 454. On one of the dyno runs they picked up 1-2hp when comparing the xtreme top to the traditional setup. I believe the article was in one of the 2004 issues. I'll put up the link if I ever find it. Even if it does hurt horsepower, I extremely doubt if you would notice it (I think on some of the dyno runs they might of lost 5hp at most). Lookin at the numbers, those tops are only makin hp with taller air filters (with the 4" tall and up) and hurt you on the smaller ones. As for the k&n filters in general, they will not hurt your engine at all, they will actually make horsepower because the more air entering your engine, the more horsepower you'll get out. Thats why when you are buying an air filter, the bigger the better, since the larger the surface area, the less resistance there is for the air entering the engine, this is why the extreme tops "should" make more horsepower. If you are seeing problems though, use a substack which helps to smooth the airflow into the carburator, thus making more power also. Then again I could be wrong, haven't took fluid dynamics yet:lol. Like I said before, these are usually small fluctuations in power, nothing to worry about and shouldn't notice anything when it comes to fuel consumption. Another site referenced the article I was talkin about at
http://www.camaros.net/forums/showthread.php?t=120807.