I am still learning, and I clearly know less than you already do. While you are waiting for an answer, could you answer a question for me?
I have learned from this forum (and also the electric bike forums) that if you go to a higher voltage, you can get the same Watts from lower amps.
This allows you to use thinner (less expensive) wire without fear of the wire overheating.
I would have thought ONLY the resistive heating element inside the water heater would get hot. If the power supply wire was thicker than neccessary, it may have been a waste of money, but would not have caused problems by allowing too much flow to the elements?
"...Do they want me to burn up my hot water heater from the inside out for running too heavy of wire..."
If I misunderstand this, I apologize, and I merely want to learn. Thanks in advance!