I'm looking to upgrade the brakes on my 2002 1.8T Quattro, I've heard that the best upgrade to get is A8 312x25 rotors and porsche boxster calipers is this correct.

Will these fit my A4

 

Well, given that the 321x30 rotors from the B5 S4 will fit, I'd be surprised if the 312x25 you're looking at wouldn't, but I also wonder if the B5 brakes wouldn't be a more significant upgrade, given their larger size.

 

Thanks for the info I will definitely be going with the larger B5 S4 rotors, what calipers should l use with these.

 

The B5 S4 calipers will fit too, and they are of course an exact match for the rotors. My understanding is that it's a fairly common upgrade, and that it'll work even under the stock 16" wheels. There are bigger brake options that'll work, but generally speaking, you need to go to 17" or larger wheels in order to clear the calipers. (I've seen kits based on Porsche Cayenne calipers that only work with 18" or larger wheels, for example - now those are big brakes!)