I just upgraded my totally soiled OEM airfilter today on my recent R52 purchase. I used the Amsoil EAAU3570 that I got from Amsoil.com directly shipped to my house for $65.15. This is the same filter as the Madness filter that I had on my previous R53 and it is $30 cheaper.

Found that critters had been picnicking in the air box and I had to vacuum it out. The foam rubber seal against the bulkhead breather had perished, so I made a new one using 3/8" self adhesive weather stripping from Home Depot. It looks better than new! This will keep it from sucking hot engine bay air. I also did the "flapectomy" per the Madness YouTube video.

Breathing, intake sound and supercharger whine all greatly improved!

 

Way to go!
I use the Amsoil filters in both my JCW's too. They're a great product and a great price.