This has only been a recent situation, in particular after BF4 and Mantle came out, NV released an updated DX11 driver a few months after which dramatically boost CPU efficiency. It was their response to Mantle. But now it carries through in many DX11 games where NV enjoy lower CPU overhead.
I suspect some of the Total System power obtained from reviews in which there's a wide gap for 780ti vs R290X even-though measured via the GPU only, they are similar ~225W average in games is due to lower CPU overhead on NV DX11. The CPU works less to push the GPU, so there's extra power savings gained!
AMD definitely needs to push for better CPU overhead on DX11 as well, without going full-retard on Mantle-only optimizations for a few games that use it.
1. It will help systems with weaker CPUs (AMD CPUs especially!!) get better FPS (Min FPS).
2. Lower system power consumption.
ps. This is a VERY IMPORTANT topic and AMD needs to be 100% aware that they have a major problem on their hand with NV being so much more efficient on DX11.