As you'd know, it'd be pretty uncommon to fry one of the cats, unless there was a fault with the engine at some stage, like a faulty coil or plug, resulting in unburnt fuel, etc, etc.
Is the code definitely for the cat, as opposed to the lambda sensor? The latter is far more common.
Even then, such 'vague' errors are not uncommon on these cars, when there actually isn't a fault! Well, one cause is low voltage from a dying battery or failing alternator/voltage regulator; but more likely the battery.
I suggest checking the battery and consider replacing it. Then use the car normally, and see if the error recurs. If it doesn't, then there probably isn't a genuine fault.
There's a Q2 currently on offer in the Tuning Parts for sale in these forums. Alternatively, visit us at ausalfa.com and we can give plenty of advice on sourcing the diff and associated parts.