Hi Mike,
Just done some quick research and the RS6 already uses a two-piece disc with alloy centre (front and rear), hence why the OE ones are so expensive.

However, the OE Pagid pads are too soft and catch fire if pushed hard (as you know ). My recommendation would be to bite the bullit and replace the OE discs, but go for the higher spec Pagid pads (not sure what compound, but if you ask here www.rs6.com they will be able to tell you the exact ones), braided hoses and some Castrol SRF brake fluid.

With this fitted, the brakes are good enough to drive round the Ring with no problems. However, you do have to adopt the "brake hard and for short periods" type of braking to protect them from over-heating, as the car weighs so much.

You can get replacement all steel discs, but they are a lot heavier and won't be as good as the OE ones (but will be cheaper, but will last even shorter).