I think that I can confirm issue with both pitcairn and tahiti GPUs, it occurs on win7 and win10, on all PCIe generations (1.1, 2.0 and 3.0).
I dont know if it happens on other gcn cards, but for these gcn1 cards, I can confirm.
Did AMD started to sabotage their own cards like nV because they want to sell us same cards with minor tweaks. Unlike nVidia, AMD did almost nothing on their gpus since they released gcn cards, except setting power limit closer to its real power consumption?!
And no, polaris have improved performance/watt because it uses 14nm, so I cant count that as architectural improvement over 1st gcn