I just installed a K&N flat panel for my HCH (part# 33-2338) and let me tell you, it helps improve my acceleration. I did not believe at first so I put the old stock air filter back and even had a friend to try it with or without K&N ( I did not tell him what I did). He was able to notice the difference as well. It's not significant, it doesn't make my car 0-60 under 5 seconds or anything, but it's a difference.

Looking at the design, as usual K&N filtration system is thinner than stock, so the air flows faster. You should do your research, there are so many K&N haters out there, even I was a bit reluctant although I had K&N filter in my previous cars.

Anyway, I just want to share my happy experience so far, the MPG however seems to be the same, may be I still have the right foot joy and drives a bit more aggresive this time.

I got mine from www.autoanything.com for $35.xx + Tax, free shipping. www.ajusa.com also has similar deal on that.
It takes only 2 minutes to replace your air filter, no tools required if you live in the bay area and needs help just holla