I have done (what I consider to be) extensive comparison between the modes. Here are my anecdotal observations:
A mode - up shifts as quickly as possible, downshifts as slowly as possible and holds the highest gear possible at the lowest RPMs to conserve fuel. Results in a leisurely power delivery and a very smooth ride.
N mode - up shifts at higher RPM than A mode, and when cruising will always up shift to the highest gear possible. Takes less throttle to downshift than A, but when flooring the throttle while already moving, the transmission needs multiple (2-3) downshifts to for max power, aka "a lag" since it is already in the tallest gear possible at any speed.
D mode - up shifts at the highest RPM based on throttle given, downshifts far quicker to raise RPM when decelerating, and does not up shift to the highest possible gear like A or N. Test this by cruising at 50 mph and switching from D to N. You will see that D cruises at a higher RPM even when not accelerating. Shifts seem to be based more on speed and not RPM. As a result, when cruising and flooring the throttle, power comes on quicker since usually only 1-2 downshifts are needed. Obviously, MPG is going to be lowest using D, but you can still come close to mimicking the lower RPM shifts of N while accelerating by keeping a light foot.
This is a small liter, turbo engine with tall gears. Now that I have become used to how it drives compared to my old Infiniti V6, or maybe the car has "learned" from me flooring it a few times, I don't have much of an issue with the "lag". Also, once again, recall the Car and Driver/Motor Trend small luxury sedan comparison, the Giulia was faster in the highway passing test (40 mph to 60 mph) than all other entries in the comparison.