The RS4 subframe bushes are solid rubber and are very well constrained.
I fitted alloy bushes to my rear subframe, probably 4 years ago now, as well as stiffening up various other bushes. Its ****. The noise and harshness totally ruins the car. I've slowly replaced them all with new standard OEM parts, and the only thing left is the actual subframe itself, which i'll be replacing once the weather gets better with a reconditioned frame and RS4 bushes.
I think lots of people (myself included) rush in with this idea that stiffer is better and that we must get rid of all OEM rubber. We then fit polybushes or whatever and confirm our own thoughts, it now handles nicer that it did before.
We forget that when these cars were new, they handled excellently. The reason you get an improvement fitting polybushes, is that the original rubber bushes were 18 years old and have covered 150k. New OEM rubber will have had 90% of the same effect, and wouldnt have added all the negatives. We have a bonus on this platform that Audi developed various stiffer bushings for the RS4 (and the S4 for some of them, when compared with normal A4 models), so it makes sense to take advantage of that benefit, safe in the knowledge that its not going to add loads of harshness and undesirable noise.
Obviously if its a track car, and your chasing 10/10ths with no care for noise etc, then by all means use them. Infact thats exactly what i plan for my alloy bushes, i'll be putting them on my B5 track car.
