Bought my S3 set up off TfsiMike on here. 
Due to a slight mishap with my 4 pot front set up (brakeline fittings) I ran for about a week with worn 312 fronts and the 310 mm S3 rears. 
I reckon it's a really great upgrade to fit the rears. 
Hard to describe, but in my view it's a bit like there's someone tugging on the back of your teeshirt when braking. 
Obviously when then combined with a good front set up, you stop better and with greater control.