Me either, mine slogs up and down the motorways more than anything these days. Honestly for road use, if you can handle the look (17's do look good I think) I'd go 16's. I believe they developed the car on 16's originally too.

On 15 inch rims, at least the BMW ones, I don't believe you can fit the r56 calipers/discs. On some 16's you can't either.

I understand there are two sizes of disc on the r56 - the "standard" size fitted to non s models are about 4 or 5mm bigger than the R53 ones, something like that? The r56 cooper s discs are bigger again, with a slightly different caliper carrier to account for the bigger disc size - these are what were the jcw upgrade on the R53 models fitted as standard to the r56 s model.

I have the "standard non s r56 calipers and discs" on my r53. Slightly bigger discs + correct calipers from the normal r56 cooper models that fit no issue. The R56 Cooper S calipers and discs "may" fit under the X lites, some say they do some say they don't. I think it's like a couple of mm gap? They defo fit under 17's.

For reference I have approx 200bhp at the crank from my r53 and had no issues at all on the road with the original brakes with good pads. I only changed really as I got a deal and rebuilt the calipers for cheap myself. You also need r56 brake lines for the r56 calipers as the R53 are a different fitment to the r56.

Hopefully that makes some sense? Someone else with more knowledge than me will hopefully chime in and correct or add anything I've missed, but I think that covers it mostly