PCI is quite power hungry when it works on full trottle. The seemed power efficiency when PCIE was going 1.0 2.0 3.0 ... was due to dynamic power control, and

PCI is quite power hungry when it works on full trottle. The seemed power efficiency when PCIE was going 1.0 2.0 3.0 ... was due to dynamic power control, and link sleep. On top of it, they simply don't haul memory nonstop over PCIE anymore, since data going to/from GPU is simply not moving anywhere.