What are the physical differences between 314mm brake system and the 318mm system other than the obvious brake disc diameter; or in another way , can you fit the 318 disc to a car that already has 314mm discs as standard?
I would love to know this too out of interest. I would however look into a EBC brake pad upgrade. Very impressed with mine. Just did a whole set all round including the discs. The 318mm discs come with 4 pot callipers. 17” genuine a4 rims clear. I don’t know for certain if you can just swop it but as it’s the same model I would take an educated guess that just carriers need to be changed. Obviously that’s a guess and would be good to find out.
Here is a post I did on EBC redstuff. I got standard disks but you can get the USR slotted disks to give the extra bite. My car has 318mm Vented front & 300mm Solid rears.