If we're talking dream powertrains, I'd say for that little hood, it'd have to be mounted sideways, thus excluding a turbo I6. But why have a turbo I5 when you can have a twin turbo V6? TT 3.0 V6 making 350 horsepower and just as much torque.
Sure it'd be making more power than the new NSX, but that's fine with me, hahaha.