I have a 2016 60 that I upgraded to the 75. When I first made the conversion in Spring 2017, it would briefly charge to 240 but very quickly degraded to 238 and then 237. That was June 2017 and as of today, probably 10,000 mi later it will still charge to 237. So after an initial nearly 5% degradation, it seems to have stabilized. Current mileage is 30,000. Other things to note:
I have done a lot of long trips and used Superchargers, but almost never charging beyond 90% (it becomes too slow in any case if you do). Probably 10-12,000 mi of those 30,000 were trips using Superchargers.
When it was a 60, I charged every night to 100% under the mistaken notion that the "battery headroom" of the software limited 60 battery was on the top end. When I made the conversion it became obvious that the "headroom" was actually on the low end not the top: 20+ miles of range suddenly "appeared" during my first charge as a 75.
After around 10,000 mi of this practice I started to notice a decrease in range. 210, 208 and finally 205. At that point I started doing 90% charges daily and only range charges for actual trips which of course is exactly what Tesla recommends that you do.
So I blame a lot of the early degradation on my "abuse" of the battery, not following Tesla's recommendations.
The fact that it has stabilized now gives me a lot of comfort that it will indeed last a long time with only slight degradation over the next 30,000 mi or so.
Something that I have also noticed, using Teslalog.com, is that driving at 70 mph with the 60 battery used to use RM at about a 5% premium and 80 mph was a 25% premium. With the 75 battery I notice little or no "premium" up to 72 mph and only about a 15% premium at 80 mph. So it appears that Tesla were playing with the displayed range on the 60 to make it appear to have more range than it actually had.
