Not really full power because of a few lanes being used by PCI-e 2.0. Hence the power consumption can't be the same.
regardless of compliance with PCI-E standards the PCI-E slot for graphic card (x4, x8, x16) should give 75w of power (max). if fewer lanes will give you much lower power then you can't use x4 and x8 slot for graphic card at all.
fewer lanes --> fewer graphics power and less power consumption than on a PCI 3.0 slot
simple as that
if fewer lanes provide less power then how will graphic card would work with slot with x4 and x8 configuration? less bandwidth throughput? yes. but less power? no. regardless of PCI-E standard used on the motherboard the power consumption of the card will not change.