The ride is not particularily good on 19s as it is with s 235/35/19 let alone with 20s or 21s.
I'm sure there was a guy on here with an S4 cabriolet who ran 20s with no problems.
Even with the 19's on mine under hard acceleration it gets a bit skitish and tramlines very easily.
As for 10.5j, on an 18 or a 19 there are plenty of American guys on Audizine who run this width on the rear, some even run an 11j
Not sure I've ever seen a monster truck A4 cabriolet !!!!!!
The other thing you have to consider is tyres, NO normal tyre place is going to keep sizes like 275/25/21 (or something like this). ((Porsche Cayenne uses 295/35/21))
I'm not sure you are going to want to pay for tyres for this sort of odd fitment, or replace the alloys within about 10 minutes of driving on the absurdley bad roads that the UK have !
Stick with 19s they fill the arches nice and proper, and wheel and tyre choice is plentiful !