Its difficult to find the sweet spot on the rpm band because this bike runs smoothly at just about any RPM on any gear.
But I am starting to feel that just below 4k, 6th gear, around 70mph the engine smooth-out a little more than it is usually.
Also I am finding the shifting is good at 3k. On the other hand, this bike shifts at any RPM on any gear. It is a great commuter bike.