Sorry to post on an old thread but thought I'd share my experience and why you shouldn't just put a cheap Cat on your car.
I had my cat stolen about 6months ago, ended up having to rush a replacement on because my insurance company dragged their feet, decided to write my car off on the Friday when I had to hand my courtesy car back on the Monday, so had to rush my repair at a general garage. The whole exhaust cost near on £800, the cat itself was about £70. Also had to shell out on 2 new lambda sensors.
Since then my MPGs have dropped from around 42 mpg to 22 mpg, and I've lost a few horses. I haven't had a remap however, so don't know if this would be necessary.
If anyone has a recommendation would love to hear it. I was considering taking it to an exhaust specialist and see what they recommend, hopefully just a bigger Cat (I don't mind throwing £2-300 at it) if it means getting the mpgs back up to around 40.