I put some S3 rears to compliment the 4-pot calipers on the front. The fronts made a big difference but I can't say I was blown away by any additional braking performance with the addition of the rears personally. Apart from the 310 disc size, the pads are pretty much identical to standard. They are still pretty good though although I refurb'd mine myself and I am 50/50 if the time and cost was all worth it.
1st person Ive read that ist too impressed on the difference the S3 rears make 
Just wanting to give an honest but personal opinion. Don't get me wrong, they look great and feel good - just not a huge leap in performance imho (which might be expected when considering that about 70% of the braking force goes to the front and maybe more with a bigger setup at the front). In fairness, I've been meaning to replace the discs and upgrade the pads to match the DS2500's at the front as I've been using the discs and pads that came with them when I bought them from SteveP (so heck knows what I've got on there!!
). Perhaps I'll feel the difference then.
