Yes, this is the reference PCB, it is a Founder Edition GPU. If you also use 8 mOhm resistors you would get 280 * 1.625 = 455W
Those resistors would not work, they are 1 - 10 ohm and we need 0.008 ohm resistors. They would also be harder to solder on.
I used these from mouser: ERJ-M1WSF8M0U