I'm going by the difference it made on my S3, even everyday use the difference was very noticeable.
I find my RS3 unsettles fairly easy at the rear.
I have the 034 bar fitted - though it's on the softer of the two settings It makes a substantial difference over the standard item.
However, it's a bit bouncy still. At least it was until I loaded my surprisingly heavy new front discs into the boot to take to my fitter. The added weight plus a bit of fuel, in my case around 30 litres, transforms the car.
I have never been one for carrying more weight than absolutely necessary (F=MA so A=F/M after all) but I'm going to ensure there's at least 50kg hanging over the back axle from now on. Suggest you try that when you get your bar fitted.