I noticed a similar effect on a recent trip I made (also around 2000 miles). At 65 in 7th gear, the engine RPM was around 1100-1200 as I recall, which seems to be a bit low. I also encountered the "lugging" effect you described, but in my case I attributed it to AFM operation. 6th gear seemed to be a better option at 65 mph. When cruising at 77 MPH (70 MPH speed limit), the car was happier in 7th gear (operating at a higher RPM).
I'm surprised that you still encountered lugging at those speeds, given that your engine RPMs would be higher at those speeds.
Same here - I was surprised the first time it snapped shut. A damped spring would be a nice future fix for 2015 and later models.
Under many driving conditions, the V4/V8 transition is not noticeable for me, but for other conditions (especially in higher gears when RPMs are low) it is notable. Still, I really like the ECO mode, and as others have pointed out under many cruising conditions higher fuel economy can be achieved in 6th gear versus 7th.