If your theory is right, those 75 kwh put itself in a awkward position: worst $ spend for actual daily usage. why spend another $9000+tax for extra range only benefit a little in road trip?
This in a nutshell is why I think we do not have an answer from Tesla. If it is top limited why should Tesla make this known when they don't have to. They can still sell the 75kwh as a full 15 kwh more and 41 more miles than the 60. 41 more miles with a 75 can be the difference for some people. But if those same uninformed buyers learned it is actually only 15 more miles daily with the 90% charge , would they pay $9000? I know I wouldn't. So I don't have much confidence Tesla will give an answer to this question and I can't blame them.
When I purchased my 60D I did not even bother to entertain the thought of upgrading to a 75 and ignored that option completely.
Mainly due to TMC and the wealth of knowledge one can learn about a car. Geez, we have engineers and wanna be engineers calculating, crunching,researching and testing all manner of the tech in a Tesla.