I can't say this happens in all applications, but on the pro mod I used to work on we had to advance the cam approx 2* farther than our desired setting because we were getting that much stretch out of the belt. Keep in mind though, this was also a blower motor with 60+ lbs. of boost....
My preference is a Milodon or Milodon style gear drive. And no, most of the bs about harmonics you hear is just that, bs.....