I Just did the 312mm brake upgrade to my 140 TDI 2007 A3. Awesome, easy upgrade. I managed to find 312mm calipers with carriers from a 170 2009 A3 on fleabay. 50 quid plus 20 quid postage. They fitted like a glove. I used my calipers as they were the same, so I'll be selling the calipers to regain some cashback, and my local friendly allparts have some great parts guys that supplied ATE pads and discs for 100 quid.
The initial bite is so much better, which I wasn;t expecting from larger discs, and they never overheat when I drive spiritedly!!!
Such a cheap mod, I would recommend anyone with 288mm discs do it.