300 Amps! How are you going to [safely] run 300 amps through an electrolytic cell?
If you try to do this for any extended period of time, you are going to boil your water away and ruin your electrodes.
The higher the current through the cell, the faster the Chlorate production, yes, but I would keep the current to a reasonable level if I were you.
There is an optimum temperature and current density (current per unit surface area of the electrodes) one might try to shoot for. Faster production is not always “better” in this sense.