The 3.0T is shared with at least:
A6
A7
Q5
Q7
S4
A8
Exclusivity means very little if you get passed by a base model every chance.
The V8 sounded good, but it got horrible fuel economy and lacked the grunt of the 3.0T in every way, be it low end torque, of high end power. Besides, with emissions heading where they are, they can't afford to keep engines based on "nostalgia" at best. The 3.0T can run 11's with a flash and pulley, and get 28-30mpg on the trip back in a 4000lb sedan. It's reliable, and proven to be a ridiculously durable motor.
PS: I don't think anyone associates "supercar" status with any S/M model cars. T Hey are mainstream cars with a little extra equipment tossed in.
Not like the S/M cars get a carbon fiber monocoque and weight 1000lbs less, they are the same cars with different engine and suspension.