Perfect timing on this thread as I need to make enquiries about getting a high-flow cat next week. Meow.
Found an excellent article on them earlier this week, which has loads of useful info:
http://www.team-integra.net/forum/blogs ... -flow.html
Seems to backup popular opinion on Magnaflow (division of Carsound) being a great choice. Also, correct me if I am wrong but I believe it needs to be a minimum of 200 CPI to be compliant with OBDII standards, so 4G/5G might fail an MOT if using a 100 CPI cat.