I kind of figured it would be basically the same as the GTX1080 PCB, but with a few(VRM) components removed. Going from 5 Phase to 4 Phase.
The thing is that the amount of memory bandwidth per CUDA core is actually higher on the GTX1070 than the GTX1080.
GTX1080 = 320Gbps / 2560 = 128Mbps per CUDA Core
GTX1070 = 256Gbps / 1920 = 136.5Mbps per CUDA Core
Just food for thought.
Simply throwing more memory bandwidth at a GPU is not always going to yield better performance. AMD has been trying that strategy for generations, and it obviously isn't working. If the GPU itself can't process any faster, then the extra memory bandwidth goes wasted. So the GTX1080, with its roughly 25% faster GPU, can probably benefit from the higher memory bandwidth. While the slower GTX1070 probably wouldn't benefit that much from the higher memory bandwidth.