Interestingly these boxes, or I should say the FWD 6 speed are very popular and in demand for Ford GT40 replica builds, where they are mated to a V8, as a transaxle gearbox, in that car's rear engine set up. So they are considered plenty tough enough. In the basic FWD A4s I think the clutch/DMF is rated to a maximum of around 410/20 nm but I think the gearbox itself is maybe tougher than that. I don't know if there is a difference, outside of the obvious, between the Quattros, diesel/petrol and FWDs, in terms of strength, though. The GT40 crowd in the States tend to source and import used diesel FWD 6 speeds, mainly because of the ratios, which I believe are different between diesel and petrol, though I stand to be corrected.