Thanks for your response.
I'm currently using Z77x-UD5H, 3770K, 980 Ti x 2.
Clover 3320, iMac 17,1 (though I tried Mac Pro 6,1, iMac 14,2 16,2).
So what baffles me greatly is, 2nd 980ti shows slot Ethernet, but I took it as a mere cosmetic.
When I go into PCI view in hardware, link speed...