There's a whole span of Audi's signature V8 with 4.2L capacity.
Which engine are we talking about? At least FSI would mark a significant change in engine chronology.
For what I know, Audi themselves ran the V8 (car model) in DTM with 420hp, and that is essentially two headers alike the early 80 competition and 80 Quattro STW cars. So yes it is possible.
How easy it is with the never FSI engines I can't tell you, but the V8 NA is a route few has taken here in Europe due access to 2.2T as alternative in most offerings where the old V8 was as well. Sorry.
Classic tuning parts should be available for the 1.8 16Vs, and you need the double.
Cams at 284 degrees etc, ITBs, pistons for weight/friction/compression, flow opt, probably exhaust manifolds etc may get you there...