You might as well pony up and spend 75 grand on the RS4 in the future if you're thinking of sinking 10-15 grand into a stock S4. And you won't have your warranty voided. But that doesn't help if you're thinking of buying used, or can't wait another 6 months.

 

A RS4 would trap no where near 125MPH

 

According to R&T the 07 RS4 takes 16.6 sec. to get to 120. I'm a fan of the A4 platform though, used to own one right before I bought an Evo