The RS8 survived and he rebuilt it, but it highlights some of the potential challenges. Its one of the reasons I think the 4.0TFSI would work well, subject to the downpipes clearing the firewall. With the turbos in the V it not only makes the engine much more compact (so better airflow) but it is designed with heat management in mind.
Superchargers are much simpler overall for a small power increase but need a lot more fabrication so it depends what the goals and budget are really.
How fast can you afford to go?
You're right though - at some point it makes more sense to buy an AMG Black series!