X8  pci-e 3.0  is the same speed

as X16  pci-e 2.0  so it depends on the card.

Yeah, and welcome to the age of 4th generation PCIe (PCIe 4.0). The Aurora R7/8 support up to PCIe 3.0. The limiting factor at issue is that the two GPU slots only have 8 lanes. If you were to buy a 16x PCIe 2.0 GPU (and for high end GPU's, I think that would be circa the time of the GTX 590 or 690) the bandwidth is still limited to the 8 lanes supplied by the R7/8 motherboard. So in that scenario, the speed of (8x PCIe 2.0) is the same as (4x PCIe 3.0), in other words, 25% the theoretical limit of a non-neutered 16x PCIe 3.0 2080ti. Of course, in a real life scenario, the theoretical speed limit of 16x vs 8x is unlikely to be the limiting factor on performance.