The increase in revs that you are seeing is not from the esc with everything set to zero thats from your motor as believe me when i tell you no motor has zero timing even adjustable they all have some sort of fixed timing one way or the other nothing is zero as they would be slugs .
That does not explain why i see a increase in revs from firmware 323 to stockfirmware when both are set to zero.
I have seen somewhere that a Hobbywing motor has about 24* of timing built in.
Do you mean that the 323 firmware chooses to ignore this 24* and a stock firmware chooses to apply it when both are set to 0?