Milltek downpipes and catbacks, alcons, stasis SL, chip.....cost = about 9500 prior to install...you'll get the cool little stasis badge and have a car that outhandles the RS4, sounds just as mean, has a little more low end tq, and is just a little slower...The cost will be about the same as the bath you'll take on your S4 trade in....

keep her for a few years and see what else comes out. You will have a sick car..The RS4 platform is already outdated, and the interior far behind the S5. That said, it is a phenominal car right out of the box...

2007 Black RS4
Stasis SL
275 RE-01Rs
Hotchkis Rear sway
Strat Short Shifter
Dual LPP Jammers upfront
GMG Catback
Ferodo pads