I had a set of Coupe Brembos which required 315mm disks. I spoke to the guy who put the 2.5 24v in a 145 who had done this conversion and he said you need special disks making and also calpier carrier arms etc. The kit to fit these he said was £700!!!! At which point I promptly returned the calipers to their original owner!!!
Not worth the money to go from 284mm to 315 I thought and didn't fancy specially made disks on the car as when these wear, they will cost a lot!
I wasalso in touch with Alfa dealer mechanics on this and the 3.0 GTV has a different 5 stud hub, and they reckoned you'd need this.
Not fully convinced that it should cost this much to do, but it was enought to put me off! Best bet is to get the whole hub and caliper assembly froma Coupe. I'm sure it will fit somehow!!