I have done the Fogged air box...to Shawn's specs...and it works great. I priced an iar box bottom half, just in case I ever wanted to return it to stock. I can't remember exactly what te price was, but it wasn't much. I do work in the parts biz so I get a little price break, but it wasn't that bad to start with. The difference in the way the car runs far outweighs the destruction of the air box. Cut the thing open to 4" and enjoy it :) As far as appreciable difference in performance, just ask those 2 pesky Miatas that I keep trouncing at the local SCCA autocross....