Hi Alex,
Sorry to hear about your cat's!
I had exactly the same issue about a year ago. Threw the car into limp mode, etc, etc. I (carefully) took the car into Graypaul who had been looking after the car sinse I purchased it from them the year before. Very frustratingly just a bit more than a year ago, and so very frustratingly just out of the warranty.
From what I understand the primary cat breaks up and throws all the **** into the secondary cat. Initially it'll sort of drive okay, but basically it's on it's way out and there's nothing you can do to stop it.
They quoted me a truly stupendous amount to replace just the one failed cat, at which point I played merry **** and got them to reduce it considerably (given they'd been looking after the car since new I think they at least felt some responsibility).
Along the way I'd been looking at Larini back boxes (amongst others) and so was aware of their cats, so I called David, who in my opinion is a proper top bloke. Sadly their web-site isn't the most instructive, but he emailed me all the necessary supplementary information.
Basically between the reduced price for one OEM cat section (with no guarantee that the other wouldn't go the following week), and a pair of Larini's (with lifetime warranty by the way) I went for the Larini's.
I think the 2 cat versions are US spec', where the primary cats are incorporated onto the exhaust manifold (can you imagine the trouble and cost if the primary goes on one of these!), so go with the 4 cat, but check with David.
The OEM cat is old tech' and is very restrictive in comparison the Larini. I'll let David do his sales pitch and give you his opinion as to the quality of the engineering design of the original, but least to say it's not complimentary!
Dead easy to fit; at tops an hour at your local indi'.
Give David a call. At the very least you'll get the benefit of his opinion, even if you decide to stick with the OEM replacement.
Oh, by the way, the Larini's sound better too! Gurgle, pop, woosh! Lervely.
I hope you manage to sort this out with minimal fuss and expense and can get back to enjoying your car.
Cheers, Emtee.