D2 A8 brake conversion advice
 

Waste of time IMO unless you are towing stuff or tracking the car. Are you running a dedicated Summer performance tire?

What year is your A8? D3 360 mm calipers and rotors will bolt right up to a Facelift A8 (my 2000 - 2003) with the alloy spindle. Super cheap too. Earlier A8's with the steel upright/spindle won't work.

Best to get the calipers off e-bay or a pick a part because it will include the mounting bracket and the little hardline in the back. Those litttle bits add up. Zimmerman makes a reasonably priced rotor.

Here's one on e-bay were you can see the little pipe-to-hose bits on the back; D3 Front Caliper
Here's a set, not sure what's on the backside: set of used D3 calipers
And here's a guy who just posted a set for sale on another site: https://forums.quattroworld.com/a8/msgs/111656.phtml

Brake Upgrade
 

Replace rotors with a improved grade (VI) of Cast Iron/Cryo Treated (especially the fronts.) Upgrade to the pad compound you think will do the job (I went with the Yellow Stuff.) Then.. Pray the SOB behind you has a good braking setup for obvious reason.) This set-up costs very little more than what is available from AUDI.
The stock brake system is more than adequate unless you plan on tracking your AUDI.

Are the D3 calipers better then the S8 Brembos? I like the additional 15mm of rotor but not sure I want to throw the Brembo calipers away...

Another thing that helps is the brake block...or something that can work as one...less flex and you're able to almost lock'em up with the ABS still functioning...

The Brembos are a great setup. I'd keep them. Also the D2 S8 caliper mounting bolts are parallel to the wheel face, whereas the FL A8 and all of the D3 brake mounts bolt in perpendicular to the wheel face. You would have to swap to a FL A8 spindle and swap out the two lower control arms in order to use D3 brakes. Not really worth it.

I appreciate the incite! I'll stick with the Brembos and not complain...