Here are some pictures of my A8L rear brake upgrades:

<a href="http://albums.photopoint.com/j/View?u=1272314&a=9419231&p=32429667">Stock A4 rotor vs A8L rotor</a>

<a href="http://albums.photopoint.com/j/View?u=1272314&a=9419231&p=32851070">A8L rear brake already mounted on the car</a>

<a href="http://albums.photopoint.com/j/View?u=1272314&a=9419231&p=32851129">Side view</a>

<a href="http://albums.photopoint.com/j/View?u=1272314&a=9419231&p=32851064">Behind the wheel</a>

<a href="http://albums.photopoint.com/j/View?u=1272314&a=9419231&p=32851068">My front A8 brakes behind the wheel.</a>

Why did you chose Audi OEM stuff? Why not just buy aftermarket ones (stoptech, brembo..)? Is it because of the price? Or ease of install?

There are other reasons as well:
1) Easy installation, everything bolts on.
2) You use your stock pads, which means they get replaced for free under warranty/maintenance.
3) Parking brake retained. You loose parking brake with most aftermarket solutions.
4) Both, front (312mm) and rear (269mm) A8 brakes fit under 16" stock sport wheels.

How can you keep stock pads if rear rotor is 10mm thicker? And what about the mounting brackets- where did you get these- for the front or the rear?
Thanks

Because you use different wider caliper.But rotor and pad contact surface remain the same.The bracket from audi by modify with mechining to make it fit A4 and S4 rear mounting point.