Thanks for finding this. Would you say the difference is not significant then? I'm not an expert on CPU performance and would not want to pay $200 more for something I won't really notice. I do intend to play games on it so I would be getting the version with the NVIDIA graphics card.
Unless your a power user I think the i5 version should be just fine and I don't think the i7 version would give a significant performance boost in gaming , with DX12 around the corner the impact will be even less.
Now I could be totally wrong about CPU performance but it might also come down to how well the chassis handles thermals and if it can run cool enough to not throttle under heavy load.
The GPU was jointly developed by the Surface Engineering Team and NVIDIA specifically for the Surface Book, it has the basic core of the 950M IIRC but is also heavily modified and optimized for the device. It will game but its tuned for Video Production, 3D modeling , etc.