Hiyall, just a quick follow up years later. The racetech rotor is still working fine as far as I can tell, have had no further charging issues (...although I must point out, my XS does have an easy life these days!).
The funny noise is something to do with the drivetrain and I really suspect the rear (disc) brake. It started doing it again after I'd adjusted the chain tension, and due to the way the holes for the locking nut on the main rear wheel shaft line up with the castellations on the nut, I'd done up this main wheel nut very tight - tighter than I wanted to. I just slackened everything back off, cleaned the disc a bit in case there was a bit of grit causing the problem, readjusted the chain tension, and put everything back together but didn't tighten the main nut as brutally as the time before (...offset by one castellation of course, so the splitpin could still be fitted). The wheel nut was now a little looser than I'd like; but hey, life isn't perfect! The noise seems to have gone away again OK. Don't know if it was just grit in the disc, (...did not observe any, but my eyesight ain't what it was, even with glasses ... must be the engine vibes ;o) or if the overtightening of the rear axle was involved.
59Tebo, I doubt the rotor weight would be sufficient to affect vibes much; at least, unless its balance was really drastically out of kilter.