What
@suj says is correct in my opinion.
A lot of people retain the original air filter set up as there is not much on the market that can improve on the OEM system for power. You can replace the OEM filter but you are unlikely to see any power gains.
A K-Tec kit will give you some entertaining noise. I had one on my Clio for about a year and really enjoyed the noise the engine made when accelerating.
I have just put in a V6 airbox. Rumours suggest that the V6 airbox gives you a tiny power gain but perhaps more torque but I have no evidence to back up this claim. A V6 airbox makes a nice noise as well. It cost me about £480 in parts to move the battery and put in a V6 airbox. If you get a garage to do the battery relocation they will charge anywhere between £300 to £450 for just that part of the work.
A K-Tec kit will cost about £220, but you can probably pick up one second hand for less than that.
Or if you have deep pockets you can go for a R3 set up.