Well, as far as the K&N is concerned, I believe the cons outweigh the pros. It does let more air pass through, but also lets more contaminants in as well (not to mention the MAF clogging problem, which I've personally dealt with).
I have had really good luck with the AmsOil air filter. It's a dual-stage sponge-type filter. It traps more contaminants than a K&N (much more) but allows almost exactly as much airflow in as the K&N does. There was a website that did a dyno analysis of the two, but I can't remember which one (www.gadgetonline.com maybe?).
Also, I've heard that headers do help to some extent, but unless you plan on supercharging your ride, the only major difference is the sound (the stock headers are very well designed as it is).
Hope I helped! <img src="/forums/images/graemlins/kewl.gif" alt="" />