And here we have it after months of waiting until the right time to get it done : the 310 (and presumably 312)mm brake set up fits cleanly under 6-spoke SE 16" rims by at least half an inch clear air outside the caliper. I also have new brake discs & pads (standard 256mm) fitted to my project car a week later on the same rims, both sets fitted using braided WG/Hel brake lines, which means I have a direct comparison to how they were previously and the bigger brake setup.
Firstly - in normal commuting - the big brakes feel more progressive - there is very slightly greater pedal travel range as predicted by
@DuncanA2 above, but it feels to me as if there is more precision / feel over the pedal movement range. Ironically the pedal now feels almost exactly like the one in my A6 which has 312mm brakes. On my project car, the key thing is more firmness and bite to the brake pedal (unsurprising given the state of the discs / pads that came off it). Both are an improvement in perceived pedal feel, but in different ways.
Herewith a photo of the new setup while I was swapping to winter wheels at the weekend ; they are barely visible when the wheels are
in situ which is what I was looking for - I just can't get a good angle to take a photo with the wheels on right now owing to driveway occupancy.
View attachment 88162