A performance car needs lots of air in quickly, and needs to get rid of it quickly, hence air filters and a big bore exhaust are usually used together. Firms like K&N who specialise can produce better equipment than the manufacturer. Improving air in and air out is usually needed to make the most of other engine mods, and on their own would make a barely noticable difference.
In theory, a K&N filter is by far superior to the standard equipment, so should not affect the warranty (you would be improving filtration etc) however all the dealers I have come across are scum and will go out of their way to deny you genuine warranty claims. Modding your car just gives them an excuse to fob you off even more than usual.