well according to my H&R product list (got it from H&R guy on the Motorshow in Essen) there are 2 different sets of springs for the S3.
The comfy ones and the sporty ones.
They both set the car a bit lower, however the dynamics (if thats the correct word) are different. The guy advised me to go for the comfy lowering springs because the sporty ones are really really sporty and not lower compared to the comfy ones.
If your goal is to lower the car but not to make it bounce, the H&R comfy springs should do the job. AFAIK Eibach do A3 springs, but not S3 (correct me if I'm wrong). Because of the fact that the S3 is quite a lot heavier compared to a normal A3 I would think that Eibach would not be the best option for the S3.
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