In response to the CPU usage of the game : all along the tests i did, the benchmark only ever used 1 CPU core, and frequency wasn't even @ max. (Big ass i7 5960X 8C@4.4GHz/4C@4.5GHz/2C@4.6GHz here, but it doesn't affect how the game use CPU, it's only ever one core, frequency will not be a good comparison between AMD / INTEL, but the cpu usage is more bandwidth than actual operation on the cpu from the game/benchmark)
But your CPU can be a bottleneck for your GPU.
Regarding 2.0 Bench vs 3.0, did you do both tests today with the same settings (DX9 Maximum for a fair comparison) ?
You can observe a small performance drop with DX11, for a small quality upgrade.