I've got one RX480 attached to my old MCR320 Swiftech rad (the rad comes with 3x 120mm fans with the integrated MCP35X pump and reservoir).
The VRMs on SM boards tend to be overheating. I had to RMA my previous SM board because two of the VRM chip was on fire when I just started testing the board (yes, it was burning with fire and smoke on the mobo!)
I was a bit freaked out after that incident so I now place several Corsair fans on top of the board. They are doing a good job to dissipate heat from the VRM. The NB chip & sys temp are always around 45-50 'C with the CPUs floating around 40-46'C.