I think installing the twin turbos with intercoolers, electronic updates for fuel mixture and timing control, etc would be a nightmare. Maybe you could get one of the rear-mounted turbo kits adapted-- I think it would cost less in the end. Not much room for any other forced induction solutions, or gearing to provide the illusion of better acceleration. Still seems to me like Nitrous with a nice controller is the best/only game in town.

 

Nah those rear mounted turbo set ups are a mistake on an audi, way too much lag.
I wouldn't add turbos to a standard 4.2L either. A supercharger or buying the RS6 would be the only way to go. I personally would wait for the PES supercharger to come out.

 

there are some rs6 out there but they come with so many problems... centralizing around the trans--- id stick with the 4.2 it has decent power and its reliable