The 2.7t is a great engine.
In various parts of Europe they've taken it to over 550whp with bolt-ons. A single turbo upgrade is the best mod on these engines (but you have limited space on the S4, and it's much easier done on the A6). It gets costly at these power levels if you don't do at least SOME of the work.
Also, the 100% stock 2001-up 6spd awd trannies, axles, driveshaft and rear diff in the S4 and A6 2.7t have seen over 1000whp numerous times without failure, making it one of the (if not THE) strongest awd drivetrains available.
I've grown up around Audis, Porsches and VWs (so I know "a little" about them, and have my reasons as to why some of them make sense to me).