I've put in about 25K road trip miles on my 2015 S85D and in the early days I tried to measure the difference between normal and low suspension energy consumption. I could not detect any meaningful difference in energy usage and so left the car to ride at the default height for comfort and safety. (If there's debris in the road, you're less likely to damage something at higher suspension levels.)
Now with my MSLR, I notice that the car actually raises the suspension to improve ride quality on particularly bumpy sections of highway, which I *do* notice. I haven't ever seen the MSLR lower automatically to low, although it's possible I might have not been paying close enough attention. I will be taking my first road trip in the MSLR in about a week, so I'll play with the suspension height and see if I can detect a difference in this car, but I'd be pretty surprised if it made much difference in the new S, either.
I wouldn't worry much about the energy consumption of the various ride heights—it certainly makes less of a difference than 19" vs 21" wheels, or aero covers on vs. off. If you're needing to stretch your range to reach your destination, forget about ride height and just slow down or get behind a big truck and set AP to follow it at a reasonable distance. Those will save you FAR more energy than the ride height ever could.
