The RX480 killing mobos really isn't much of an issue for miners since we all use powered risers. The ribbon risers usually have a thick 16AWG cable connected to the molex, I am sure this cable can handle 100Watts no problem.
However no idea how those USB risers will work since they are powered by the SATA connector and those usually have a smaller gauge cable then regular molex.
One cable will handle 100W without problem. But do you have 6 molex cables connecting to the power supply?
That's true, there is only one +12V line on the molex connector. And most PSUs have 2 cables and each cable has 3 molex (6 molex per PSU). But if they actually use 100Watts then one PSU can only power 2 of them.
You would have to ghetto mod the PSU and plug the riser +12V into the CPU plug which should handle another 400Watts with the 4 +12V it has. Assuming your PSU has 2 CPU plugs.
A big headache, might as well wait for a 6+8 pin RX480 or 8pin RX480 from another manufacteur.