I have an ASRock 970 Extreme 4 and some shit 170u CPUs. I have 4x Zotac 750Tis in my rigs and they don't slow down except for thermal slow downs. But then again my cards average around 280-295 to begin with.
I don't think I saw many people with AMD having a problem because the motherboard is still controlling the pcie spec. With intel it's mostly controlled by cpu now, so even if my board theoretically should be at least pcie2.0, the cpu is going to run it at what it is programmed to. At least that's my theory. Maybe AMD guys were having the same problem, but I'm pretty sure their only problem was getting multiple cards to run properly on certain boards.