Going back to basics here I think the reason Audi have not brought out (for example)an RS4 diesel is quite simply because no-one would buy the petrol version. I mean a 4.2 TDI with suitable tall gearing and all that torque against the high revving much thirstier 4.2 petrol... I know which I'd go for, especially with fuel prices sneaking back up again...
I do love my petrol 20Vt in the Cq though as it
can be driven like a diesel (max 295lb/ft torque at 1950rpm) but can be revved upto 7000rpm if the mood takes me passing most modern cars in the process. The main downside is that if I drive it this way it reduces my mpg from 30 to 12mpg.
so as the month progresses it gets driven more and more like a diesel until I get paid again, then it's fun time all over again...
My tuned Passat TDi average on the other hand never drops below 30mpg no matter how hard you drive it.
If Audi do bring out an R8 Diesel, I think they probably realise they'll lose 90% of the sales of the petrol version.