How to choose transmission ratios?
 

Possibly a stupid question, but for the track I understand you want gears for the corner speeds you'll be at.

For the street, why is a progressively closer gearing (say, 33% drop to second, 26% to third, 23% to fourth, 20% to fifth, 17% to sixth) optimal over a steady gearing, like 25% drops across the board?

I think you have to work with what is available and make it work. you can look at flyinMiata calculator https://www.flyinmiata.com/tech/gearing.php

Yes. One does have to work with what actually exists. That doesn't answer my question, though.

because the torque multiplication drops too and your car will feel like a dog

Aerodynamic drag becomes a factor, hence gear ratios get closer.

This is the correct answer