The differences may not be significant, but there are absolutely performance gains or losses based on motherboard model alone. Consider as well, that comparisons between lower end or mid grade boards, and a high end top shelf flagship model are generally a bit wider. I've seen reviews using otherwise identical hardware where just the board itself made as much as a 5-8 fps difference on some titles and the performance differences were even evident in productivity application benches and synthetics. I'd have to really do some digging to find those Z170 reviews I'm referring to, but if needs be I can do so.
The idea that aside from overclocking, all boards are a level playing field is just not accurate. Of course, we're not talking GTX 1060 vs 1080 type differences, but there are differences other than simply how well a board can overclock or what kinds of storage is supported.
https://www.kitguru.net/components/motherboard/ryan-martin/asus-rog-maximus-x-formula-motherboard-review/9/
And definitely how many phases are set aside for memory, cpu, etc., can certainly have an effect on component performance.