Yep, engines don't like vacuum leaks, do they. Eliminating them makes a difference. But, these leaks can sneak up on you as they keep adding up slowly.
Honda gives a pretty wide spec [ 1.6 in/hg] on the sync, and this system is pretty bullet proof, but, a sync may help help the idle and transition a bit smoother.
Patrick - you are spot on. I did a digital sync on The Valkyrie yesterday. The idle is much smoother and I had to wind back the idle speed knob to counter an increase in idle speed as the carbs got closer to a perfect sync.
After taking it out for I ride, I can fully support your observations. She idles much smoother and the transition from idle to putting the power down is also much smoother. There was a lag or slight hesitation there which only became apparent in its absence following the sync.
This bike has never run better!