I must be missing something. I never thought that putting in a transformer halfway had any benefit unless you needed to tap off at that point. I always thought it was better to put the step up transformer at the start of the run and the step down transformer at the end.
The longer the high voltage portion of the run, the less energy lost in voltage drop given the number of amperes needed at the final voltage.
Isn't 600 volts hot to ground and 1200 volts hot to hot classified as a 1200 volt system? An aside: the voltage rating of a 3 phase system is the voltage from phase to phase.
You're right. Article 300 uses nominal voltage, not voltage to ground, to define the insulation class required. So much for that idea!