Personally, I cannot detect any audible change between different cables—the portable context makes it even less likely that I’d be able to gauge any nuances that may/may not be there.
That said IEMs rely considerably on a proper seal to sound as there were intended. I have found that changing the cable can improve the ergonomics of the IEM, which improves the consistency of a seal and therefore has an audible benefit. Aside from that, I’ve bought some cables with poor connectors that caused audible drops.
I know that’s not the answer you’re looking for, and I’m smart enough not to venture into the cable sound debate, but that’s my honest take on the “audible” benefits of different mmcx cables!