Mantle has the same CPU savings, and during discussions on that game developers have talked about how in many (most?) games, up to 50% of your CPU time is spent in the drivers. Considering how many games I've run into lately where single-core performance is the bottleneck, this doesn't surprise me at all.
Mantle and DX12 strip out a lot of layers, optimize a lot of what it keeps, and spread a good chunk of the remaining load over multiple threads, meaning that instead of trying to use 120% of core 0 and 15% of cores 1-3, you can use 60% of core 0 and 25% of cores 1-3 (just as a hypothetical made-up example).
