Why.........
Take one knackered cat, remove the pre cat, cut out the main cat and weld a sports cat in place. To ad about 12Bhp (from what i have read)
When you come to sell the car or there is a problem with the emissions (s#it happens), the original can be replaced.
Ive seen the whole assebly go for about £400 and the sports cats on Ebay for about £80.
If i get the parts i will take some pics and let you know how it went.
Car.crash you have a p.m!