ounce for ounce
Good point. The Skylake processors have a range of speeds and number of cores. I think the "twice as fast" remark involves the discrete GPU and that none of the models have a 4 core CPU because of high TDP.
My guess is i5-6300U @2.4GHz and i7-6600U @2.6GHz (both 2 core, both with Intel HD graphics 520), and Nvidia GPU equivalent to a GeForce 850M (GM107, but maybe a newer GM108 model that can handle GDDR5 memory), partly based on what Ryan Shrout says at http://www.pcper.com/news/Mobile/Microsoft-Surface-Book-2-1-Skylake-NVIDIA-Discrete-GPU-Announced.