To expand on what ARob said, by running the OEM EVSE at 240v you're not changing the amperage it outputs. That stays the same no matter what: 12A. The wiring was designed for 12A and it won't output more. At 240v it doesn't put out 2 x 12A, that's now how things work.Sorry if this is redundant, but is it impossible to use the OEM EVSE at NEMA 14-50 and achieve 30A charge rate? Is it still limited to 2x12A?
120v * 12A = 1440W
240V * 12A = 2880W
So doubling the voltage doubles the wattage (power), not the current (amperage). The only way to get more amperage than 12A (and also more wattage) is to buy a different EVSE that will output more power to the car.