To maintain the same relative body to wheel positions for the MR sensors, you'd have to keep the the same circumference for the tyre/wheel combination. This means you would have to mount 255/30/20" tyres on your 20" wheels to keep to the 2076.9mm circumference of the stock fitments (based on the 19" RS4's). The Oettinger 20" tyre fitment is 245/30/20 which yield a circumference 1.9 cm smaller than the stock tyre/wheel combo. That makes for a mighty thin sidewall.....you hit a pothole or curb at even 30 kph and you're going to need a new wheel
Presumably there is some tolerance in the MR system...but I have no idea what it would be. I would suspect in the end that if would have little to no effect since it is unlikely that an appropriate 20" tyre would change the circumference much.
Lowering springs on the other hand....now that's a matter of some debate. :wink: