As an owner of an RS4 (and it used to be two of them), for racing I think the biggest weak spot of the RS4 is the suspension design. The car uses dogbones or turnbuckles for upper arms front and rear. Getting good suspension settings and good steering response from this setup is harder than from a design with upper arms, such as the TC3 or R40. But the average guy won't notice it. I suggest CVD's for the whole car, especially if you plan on keeping the 18 in there. Drive dogbones on the RS4 are small and get lost when the suspension dogbone links pop off in a crash.
This will sound crazy, I know, but I find the car easier to drive due to the fact that my Futaba radio has an anti-lock brake feature that pulse the brake servo. It just works for me.
I got wide foams for the car. Watch the wheel offset. The rears can hit the radio box when you get around 30mm wide or above. I ground away the rear corner of the radio box to get more clearance, but could have spaced the wheels on the hexes, I suppose.
Ditch the stock body if it has a viper. The Toureza body is more like what you want, or a Mazda6, or Stratus.
Ditch the drycell holder and buy a rechargeable pack for receiver/servos. The car will drive more consistantly over time, and cost less to run, over time.
Other than that, play with the shock oils, springs, tires, camber and toe settings (if you have turnbucles) to get it to drive how you like.
OH....buy a cool flag painted BMW body for it!!! see for sale section here.
shameless, I know.....