I have a Mac Pro 2010 (5,1) with an EFI Sapphire 7950 (boot screen) running. Everything seems great except I wondered why the system report shows the PCIe lane width as 4x. This machine has 2 16x slots for GPUs. I have it in the so label 16x slot. I have moved this card around to test with no change. I have no idea why this would restrict the bandwidth of the card unless it's a bug. Machine running High Sierra.