Agreed the GPU going from 16x to 8x is not that much of a performance hit, but the DMI bus will be a bottleneck for SATA, ethernet, USB and NVMe SSDs together. The DMI 3,0 link is equivalent bandwidth to just 4x PCIe lanes. The new Samsung NVMe drives do 5.5GB/sec reads. That pretty much uses up your DMI 3.0 bandwidth.
I've very well aware how much bandwidth it has. I got the numbers straight from Intel themselves. But my point still stands. You won't normally saturate the DMI bus because you won't be hitting the maximum bandwidth of the network controller, NVMe drive and USB ports. You may do one or two of these at a time on occasion, but not all the time.
USB 2.0 devices are basically inconsequential. Even if you were using USB 3.0 or worse yet, 3.1 devices, I doubt you'd be maxing out their transfer rates. If you connected some type of external storage and again were doing large transfers then maybe. But again this is only a temporary issue. If you look at your network traffic, unless your doing large file transfers, you won't be hitting 1Gbps speeds. I've never seen a controller actually hit that outside of very controlled conditions with specific tuning in the drivers.