I'm not using this specific board, but I am using a board that provides extra molex connector which supplements the PCI-E lanes. It is also ASRock brand, but higher quality. I am running 5x R9 280x per board. I use all powered risers and the extra molex on the board. Did your comment apply only to the specific board that was on topic or all with PCI-E supplement molex? I've been successfully running 2x rigs (clones) this same way for almost 3 months now with no issues...

From what I understand, molex connectors generally provide around 75w of power per connector. 75w spread over 5-7 PCI-E lanes is not much for a power hungry GPU. Now 75w extra per card (via riser) is a good solution to get the card the power it needs. I don't think it would fry it. It's not going to pull anymore power than the GPU is asking it for. Generally PCI-E lanes are burnt out when you don't use powered risers because the card is trying to pull more power through the bus then the bus is designed to handle. This is why you supplement on the riser itself. Just because the power is available doesn't mean the GPU is using it. This is given the exception of a power surge, in which case the power is forcefully pushed into the circuit and therefore the hardware that is not designed to handle so much current which ends up frying it.

I suppose it all depends on how many GPUs you intend on running off the board what you would need to do. Personally I would always use powered risers for all GPUs.