The mobo has more than 2 year warranity. Also, in mining, the gtx 1070 takes around 140w, most from the 8pin pci-e connector?
Also more power hunry gpus like gtx 580 were on pci-e 2.0.
It depends on how the card is configured to draw it's power. Older PCI-E v2.0 cards got most of their power from the VGA power connector. Since the v3.0 specification allows for more power drawn from the slot, v3.0 cards tend to draw more power from the slot. In fact some PCI-E v3.0 video cards state they require a v3.0 slot. When the RX 480's came out, many gamers were burning up their motherboards from just gaming with the card.