Your reasoning is a little off here. Nvidia claimed it would double performance of the Titan X with the GTX 1080, but in a foot note it clarified that this was in a very specific workload. What Nvidia did was advertise its product using a statistic it came up with while testing its new card, it wasn't completely honest about the general performance of the GPU, but companies make these claims all the time and it really isn't surprising.
That is completely different than what I did here, where I can actually see all of the parts on the board and get some idea of its abilities from that. The most basic concept about how power phases relate to overclocking potential, is that more power phases leads to better overclocking results. This is because all of the power phases on a board share the task of regulating voltage. Some of the power phases may be used for other components such as memory, PCI-E, the north bridge etc., but most are used for the CPU. If the power phases get too hot, it can lead to thermal throttling, which hampers performance. Because power phases share their work with each other, the effect of having more power phases is that less power needs to run for each individual phase, and thus they remain cooler. This helps to improve overall efficiency, power regulation, overclocking results and has other positive effects on the system.
The simple fact of the matter is the more power phases a board has, the better at overclocking it is in general. Any board targeted as a high-end enthusiast overclocking motherboard should not limit itself to just 8.