I would avoid having a cat in the can if you intend doing a lot of track days. In my experience it kills them. I had a Powerspeed 4-2-1 with the cat in the can ( until i had it removed) and was having to get it replaced every other year due to emissions failure at MOT. With the new cat in it's fine. And before you ask YES this is a properly mapped DaveW emerald setup. Cats work well in normal driving conditions but on a track the engine will always be on a transient throttle or RPM. This means it will spend a lot of time breathing richer than normal exhaust gases. It burns them out far more quickly. Sooo, I have two exhausts. One with, and one without the cat. To save costs tho I asked Powerspeed to take my original Caterham cat exhaust and modify it to fit onto the Powerspeed headers. This way at MOT time I don't have to swap the whole system, just the can. Simples.