Disclaimer: Use the information contained in this FAQ at your own risk. You personally assume all risks in connection with installing and using those kits, however directly or indirectly related, including, but not limited to all risks of loss or injury to you, your property, and the property or person of others.
12.3" Front kit:

Slotted rotors courtesy of APR.
312mm x 25mm front A8 upgrade kit will work on the following cars:
A4: FWD + AWD, 1.8T + 2.8
A6: FWD + AWD, 2.8
Passat B5: FWD + AWD, 1.8T + 2.8
Parts required:
Two Audi A8 312mm x 25mm vented rotors: Audi Part# 8D0-615-301-J
Two Audi TT Quattro front caliper brackets: Audi Part# 8N0-615-125

The kit bolts straight on (D.I.Y. instructions on the bottom).
10.6" Rear kit:

269mm x 20mm rear A8 upgrade kit will work on the following cars:
A4/S4: AWD, 1.8T + 2.8 + 2.7T (No calipers required for the S4)
A6: AWD, 2.8 + 2.7T
allroad: AWD, 2.7T
Passat B5: AWD, 2.8

Parts required:
Two Audi A8 269mm x 20mm vented rotors: Audi Part# 4D0-615-601-B
Two Audi A8 caliper brackets: Audi Part# 4D0-615-425-B
Audi S4 left caliper: Audi Part# 8D0-615-423-C
Audi S4 right caliper: Audi Part# 8D0-615-424-C
Four 2mm washers: (~$0.05 at a hardware store)
Before the kit can be mounted, the caliper brackets have to be machined to clear the control arm. You need to take off about 0.5mm in a very small area near the control arm. You can do it quickly with an air powered cutoff reel. The bracket has to be spaced from the wheel hub by two 2mm washers.

Additional info

• Both kits will work with the standard Audi/Passat 16" wheels. The rear kit may require spacers on some wheels. (My 7-spoke OEM A4 16" clears the caliper by less than 1mm) 15" wheels will not fit.
• ABS/ESP etc. are fully retained, since the ABS sensor stays stock.
• Rear kit retains full functionality of the parking brake.
• Neither kit requires new pads. Both reuse the stock pads. (Which also means free replacement under Audi maintenance program when the pads wear out.)
• The front kit reuses the stock calipers. The rear kit reuses the S4 calipers on those cars. Bleeding the brakes is not necessary if you don't change the calipers.
• The original A4 front brakes are 276mm x 25mm (10.9" x 1") vented. The original A6/Passat front brakes are 288mm x 25mm (11.3" x 1") vented. The A8 upgrade is 312mm x 25mm (12.3" x 1") vented.
• The original A4/Passat/A6/Allroad rear brakes are 245mm x 10mm (9.6" x 0.4") solid.
• The original S4 rear brakes are 256mm x 20mm (10.1" x 0.8") vented. The A8 upgrade is 269mm x 20mm (10.6" x 0.8") vented.
• Rear brake weight info: The stock calipers that come with the 245mm rotors are steel and weigh 5lbs. The S4 calipers are aluminum and weigh 4lbs. The stock brackets are 1.75lbs. The A8 brackets are 2.25lbs. The stock rotors are 7.25lbs. The A8 rotors are 11lbs. The stock dust shields are 1.25lbs. The total weight increase when upgrading from 245mm to 269mm brakes is 4lbs of unsprung weight (2lbs per side.) I recommend against a new splash shield, since it has two major drawbacks: increased unsprung weight and reduced cooling of the rotor.
• The surface width of all front rotors (312mm, 288mm, 276mm) is the same. The braking surface is just moved outward on the larger rotor. Likewise, the surface width of all rear rotors (245mm, 256mm, 269mm) is the same. Stock pads will cover the entire width in both cases.
• Larger rotors have two big advantages:
  • Even though the braking surface is the same, the larger rotor has greater linear velocity with respect to the pad compared to the smaller rotor at the same vehicle speed. The greater velocity increases friction thus giving better breaking performance.
  • The larger rotor also has better heat dissipation, increasing braking efficiency. In addition, the vented rear rotor has much better heat dissipation than the solid one.

D.I.Y. Instructions for the front kit

• Remove the spring clip from the caliper using the Audi supplied screwdriver.
• On the back of the caliper you will notice two plastic caps covering the 7mm hex screws.
• Take the caps off.
• Unscrew the hex screws and gently move the caliper out of the way.
• The caliper bracket is attached using two 17mm screws (I had to use my 1.5 foot long torque wrench to unscrew them as the Audi wrench was too short for the amount of force required to undo the screws. I had very little room to navigate the wrench, but I managed).
• Remove the bracket and the rotor.
• Do everything in reverse for the new rotor and bracket. You have to torque the caliper bracket screws to 92 ft-lbs. You should also torque your wheel bolts to 90 ft-lbs.