Hmm, well that means you don't need a modded ROM, this default ROM is already 99 Watts, so should be able to have even 6 of these cards on a 1000 Watt PSU.
The only benefit I see for making a custom ROM is trying to get the VRAM higher default clocks and Lower the CORE Voltages and clocks to be more effecient.
That's weird. I most certainly cannot fit 6 of these on a 1000w. My kill-a-watt puts it over 1000. I guess try to crank down the core and I'll test it.
Here are the ROMS:
https://drive.google.com/open?id=0B6VgxfTqxux4cWFWbUpDelhQWFkI made 2 core variants and 2 memory variants
Try the Stock Mem and Core 1125 mV 850 first, that should be the lowest Wattage ROM that would be named "Asus.RX480.4096.160817_core_1125_mv_850_memory_stock_timing_stock.rom"
If it's stable for more than 2 hours, I would then try the "Asus.RX480.4096.160817_core_1125_mv_850_memory_1900_timing_1650.rom" Which is the ROM with same low power core clocks + 1900 Mhz Mem Clock and 1650 Mhz Straps for better Dagger and Equihash performance.
BTW, do you have multiple GPUs? Incase the ROMS fail on boot, you can reflash the GPU with stock ROM if you have another GPU that you can boot with. (I'm assuming you know how to do with with ATIFlash/ATIWinFlash?)
Awesome! Will test tonight and report back! Yes I have bricked cards, and re-flashed using another GPU before. I use ATIflash cmd line tool.