I plan to do some soundproofing on my S3 to reduce the road noise from the wheel arches and boot floor. Does anybody know if there is a guide somewhere for removing the S3 trim (wheel arches, doors)?

I did this with a Ford focus and it didn't make a blind bit of difference.

I am surprised that you find the S3 noisy.

You would probably achieve more sound reduction by changing the tyres as this is the primary source of noise.

I guess I have sensitive ears . I have done this to my two previous cars and there was a noticeable difference - the rear end tends to be the main source of road noise. The end result also depends on the materials, I added a layer of mass loader (STP Bomb Aero) + a layer of softer foamy material on top of that (STP Biplast). 

On my old rover I got a couple of rolls of bitumen roof flashing tape and applied a couple of layers to the insides of the wheel arches, using a heat gun and squidging it into place.      Made a big difference to water noise in the wet, and a bit of difference to normal road noise too.   Not sure if you'd be wanting to do it on your shiny new S3 though ;).