I had to do a proper full on emergency stop in the van on the Autobahn in Germany on a down hill stretch at around 60, fully laden. It stopped very well, better than I thought it was going to at the time, dead straight as well. Stock brakes. The only time they grumbled was on a couple of long steep descents, so stopping power isn't really the issue, more overheating with repeated use. Better pads might fix that, or even vented front discs, but I doubt rear discs would make a lot of difference. The only point for me with rear discs would be less maintenance, but it doesn't seem to be worth spending money on that when you could be spending it to go faster.