I have to wonder how much of that is down to the way the driving habits of people who buy a car labeled "long range" differ from those who buy one that calls itself "performance". It doesn't really refute the notion that the difference between measured and claimed performance is due to the different driving conditions being tested.

IIRC there was some youtube video a year or two back that tested a few variants on a track using what they claimed to be a replica of the EPA highway test, and the cars came out almost dead on.

AFAIK, Tesla uses the same range estimate for all trim levels of the Model 3 (or did in 2019, at least). The problem is that the Performance trim has 20 inch non-aero wheels and sticky tires. I got 325+ Wh/mi steady state cruise on the highway using AP to do the driving (in good weather; was 350+ when colder).

There have been some independent tests and IIRC the consensus is that the standard range Model 3 was rated pretty accurately, and the Performance trim was rated something like 25-30 miles too high compared to reality.

The current site lists the LR at 353 miles EPA range and the performance at 315.

But again, you're using the words "on the highway" to reflect what I assume to be your driving habits in rural open road conditions and not the test that was actually performed to produce the numbers in the product specs. And that was my point. It's not about advertising fidelity at all, it's about regulation and its mismatch with the expectations of a modern EV driver.