Sooo....
A8 upright, A6 C5 hub, OE S8 345mm disc:
To make the S8 disc work on the A6 hub (moving it inboard 6mm) means you forfeit the splash guard, and I think the hub needs a little 'lightening', although it is possible to offset the hub up to 2mm with a laser-cut aluminium spacer so that might be a solution. The 18z caliper fits fine with a spacer washer (maybe offset by spacing the hub) and the usual bolt adapter set with a sleeve for the caliper side. The pad may overhang the disc slightly (since its 2.5mm too small), although I've seen mention of using 17z pads in 18z calipers to solve that. Still, with M12 hardware it would be possible to move the caliper in a couple of mm.
That gave me an idea...
A8 upright, A6 C5 hub, D3 A8 360mm disc:
This also forfeits the splash guard and definitely needs a mm or two shaving off the upright, as it is exactly centred in the caliper as-is. Its very tight on the caliper and may need a little massaging of the caliper inner radius (or machine 2mm off the disc). Again, with 12mm hardware it might actually be possible to move the caliper out a little.
Alas, none of the lightweight discs I have are even close to fitting as the offsets are completely wrong. It could be done with custom alloy bells, but thats missing the point.
I'm not sure what the pad radius of the 18z caliper is so I've ordered the cheapest Q7 pads on ebay (£26!!) to find out. I will report back at the weekend when they are due to arrive
