If you have to have a Cat, then the SARD is really your only option. Compared to the stock Cat, I feel that it is head and shoulders above. No comparison. Spool is great, response is great. Your car just feels like it just had a massive sinus conjestion removed. I can't compare to a straight pipe, since I've never had one before, but I am more than happy with my Sard downpipe+cat.
My wallet was in great pain for a while though. You can buy them from Japan. I can help you get one easily, or if you have other friends in Japan they should be able to easily call up SuperAutobacs or Sard themselves, and order one too (assuming their Japanese is sufficient).
My email is
vaeth_fv@yahoo.co.jp if you want to contact me about getting one.
--V