The S4 2.7tt has 265 bhp,RS4 has 380bhp.The Rs4 engine was developed by Cosworth and has a far superior head design with performance cams,larger higher flow intake system,K04 turbos (as already stated),freer flowing exhaust and bigger inlectors.
But it doesnt stop there.The suspension is much stiffer and better damped for improved handling.The brakes are huge,360mm rotors up front compared to the S4 with ≈330mm.
And yes,it only came in an Avant.The USA never received the RS4,most of the rest of the world did.Here in NZ we have about 20 or so,with a similar number in Australia.It is Audis highest performance road vehicle ever produced.Slightly more so than the RS6!