Installing a boost guage is relatively straightforward. The problem on the S3 is where to locate the guage. Some people have mounted them in one of the air vents, but you obviously loose the use of the vent, I have also seen some mounted on top of the dash, but again that leaves holes.
I have mounted mine on the A-Pillar using a Revotec holder, however I also bought a replacement A-Pillar cover from Audi (~£17)so that when I sell the car there will be no mounting holes showing.
Hope this helps
Regards