Oh, the label on the stock fan serves as compliance label for the whole device, it was custom made for us. So 5V/0.8A is for the whole camera. The stock fan also consumes about 0.2A.
Mostly, it is about aesthetics, the thicker fan provided with the global shutter kit fits the larger case better. Also, since the stock fan is glued, it may get damaged during the conversion to global shutter, that's why we included a spare one in the kit.
But if your stock fan works fine after removing the stock case, then you can keep it, the air flow for both models is the same.