Someone just got dansk cats on here and did a before and after dyno pull with some pretty good results. I can't think of his screen name for the life of me right now. It was recent though...within a week so you may want to do a search for the post. Have you ever considered just doing away with one set of cats completely? Here is a link to a cheaper and possibly better alternative if you aren't concerned about emissions regs. Click on boxster and check the bottom of the page.
http://www.fabspeed.com/