Tony,
K&N Filters can increase the Aural Quality of the induction - although there have been many reports that show no actual peformance gains...
Removing the Foam from the OEM filter also helps the sound experience in a similar way.
MAF Sensors used to get clogged/oiled up by installations of K&N (or other "oiled" filters) if they were over-oiled. Wrapping a newly oiled filter in a towel overnight is said to help mop up any excess (and then put the towel back in the airing cupboard - no-one will know...)
Revised MAF design has alledgely reduced the problem on later cars.
Other routes exist - including "de-snork" mods which removes the trunking leading up to the filter box. Makes noise but again unlikely to increase performance (the opposite is probably true[8|])
Removing airbox altogether and using direct-drawn air just means you are using hot air from around the engine (rather than cooled from outside) so reducing performance even more...
Complete replacement systems MIGHT have benefits - only a rolling road knows for sure... but these cost a lot more. I have not seen any actual factual gains demonstrated, but that may be because I mostly hang out here now[
]
All the above will probably affect warranty. Some OPC's have said K&N not a problem, others just say "no" (and make a mental note that you have asked no-doubt[>:])
Any actual real genuin performance enhancing mods (should you ever find one!) should be advised to you Insurance company. They will no-doubt say well done and present you with thier revised premium forthwith...
