For anyone that's interested I just finished fitting a GSF Catalytic convertor to my '96 1.8 (ADR) A4 Avant today. Its probably not quite as solid as the Audi original & its a slightly different shape but it fit well (in that the two ends lined up where they were supposed to be!). Possibly slightly noisier outside the car but not noticeable inside. Cost around about £170 inc VAT (including all the brackets) versus about £320 for the Audi part.
Other than that, t'was a bitch of a job to get some of the old bolts out - made somewhat easier once I got hold of an Angle grinder!
rgds
I.