Then your previous claims of "Its not dependent on math" and that we "don't know" the sidewall size make even less sense.
it's exactly dependent on math- as far as tire sizes go. And the tire specs tell us the sidewalls.
Which is why the explanation you got from the Tesla person made so little sense.
At this point, since both tires are rated for speeds over 155, there's only 2 possible explanations-
1) It's just a software unlock (more likely)
2) There's some bizarre instability that only happens above 145 mph that is fixed by the spoiler included in the "performance" pack (this is exceedingly unlikely- but is the only, relevant, mechanical difference we know of between the 18"P and the 20"P)
Given both versions list the same 0-60 spec, obviously not.