Been following this case for a long long time, and while I WANT to downsize (ATX -> mATX) as you said, the mATX motherboard market has been all but forgotten at this point which is a shame. ITX is an option, but the NCASE M1 already exists for that, with new contenders coming out seemingly monthly recently. So, it seems futile to move Cerberus to ITX. I think Cerberus-X still has some uniqueness to it (build materials, still extremely small [for an ATX], and it's internal design geared towards the enthusiast [but not exclusively]) which could make it shine while the motherboard market being infinitely more exciting, and varied in the ATX segment. Providing people with *gasp* choices!
So, to sum it up, I support the move to ATX. Although, I'm disappointed in the motherboard industry for even making us consider this. It really is a chicken and egg problem though.