I haven't driven the RS4 yet - but owned an S4 before my 335. I expect the RS4 will out-torque the 335 to no end, since my S4 always "felt" faster to me than the 335. I loved my S4, I really like my 335. I expect I'd probably like the RS4 more than my S4.

27k miles on my 335.
I can probably seriously get around $30k for it give or take selling it private party. Less if I were to do the dealer thing.

The buyout on the RS4 will be really high I expect. I would expect the residual to likely be more than the value of the car after 3 years with 15k per year put on the car.