It's my understanding that the carb sync at high revs under significant load isn't nearly as important as at low revs. Not because you want your two pistons to be out of sync, but because at high revs / high load your throttle is almost all the way open, you're using the main jets in the carbs, and any differences between the two cylinders are going to a be very small factor compared to the power being produced by either.
When you're idling along with the smaller jets at very small throttle openings, that's when just a little difference is much more noticeable as vibration and a poorer running engine.
