There are no reasonably priced off the shelf upgrades to the existing rear callipers. Due to our callipers having an integrated handbrake most options I found use separate foot and handbrake callipers on custom brackets.
To uprate your rear brake you have the Audi RS4 / RS6 alternatives but without a matching front upgrade you will unbalance your baking system shifting the bias backwards. The net effect will be a twitchy rear end  . Non of us want a twitchy rear end  . All that will happen is your ABS on the back will work overtime as the back locks up before the front.
As Psyche says, refurb or replacement is the way ahead. Refurb is not difficult and Audi do a kit for it. Parts 21, 22 and 26. Here on VAGCAT (you need to register)
I would be suprised if the calipers a beyound refurb. If they are try A8Parts.
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Conan (the Librarian)
Watch it if you dare
Currently,
2000 S8, Merlin Purple, Leather Pack 1, Winter Pack 1, SPAX adjustable suspension, AP Racing 6 pot front caliper, 385mm fully floating disks, EBC Yellow Stuff Pads, Black Diamond grooved rear disks and predetor pads, D2 Doctors Brake Block.
1997 4.2 A8SQ - the Projekt. 12 years and ongoing!
1997 4.2 A8SQ - Ming Blue. Restoration sat waiting; saved from the Scrapyard
2003 C5 Allroad 4.2Q - Cobolt Blue. Towing car. Laid up awaiting manual conversion.
2005 C5 Allroad 2.5TDi FE - Grey - Workhorse.
2015 Q3QS SLine - Daytona Grey. Lots of extras! Long Haired Admirals cutter.
Previously
2003 C5 A6 1.9TDI FWD - Loaner from D2 Doctor Lifex
1997 S8 - Ming Blue. Loaned out and written off. I loved that car. My first engine swap after cambelt failure.
1996 4.2 A8SQ - Ming Blue. 178k on a cambelt! Trader's 8. Delivered to A8Parts at Lifex.
2003 2.8 A8SQ - powder blueish. Fill in for a while. Sold on.
1998 1.8T A4 Avant FWD. Ming Blue (there's a theme here), PEX for the 97 S8.
And a list of non VAG going back 40 years before I saw the light. 
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