used my 06 plate XT R for commuting a 50 mile round trip daily on A roads for a year now covered 10.5K miles in that time and rarely run below 80MPH up to 100+ on my private sections!!! (road that is not danglies!)
a good part of that which is described a surging will be just poorly adjusted chains any way, keep it adjusted and well lubed and its all good
MOT'd mine today and only needed rear wheel bearings, not bad for a bike which was used all but a few days last winter and has 21K on the clock
my only gripe is hard seat for longer runs