From what I’ve read so far, the Doo belt is not bad at all, just was not as sticky as the 821, making the 821 more responsive, but it dropped the rpm’s a bit so caused for slight adjustments with the clutch to compensate.
Which was interesting to hear the belt on the Mach is stickier…. maybe they changed that based on the above feedback from previous years.
I have both, so can test them both out and see if my seat of the pants feeling can tell the difference.