Frequency response is far and away the most important thing.

But frequency response curves are highly smoothed graphical representations of the response to a sine wave sweep, intended to give information about tonality. They aren't designed to measure everything.

And there are other things that can be measured, including some of the things you mentioned. To some degree we can measure distortion, speed (or delay), and also things like mismatches between L/R drivers in phase response, frequency response, and amplitude. All of which are above audibility thresholds in at least some headphones.

Qualities like soundstage and imaging on the other hand may have no one direct measure, but are impacted by both frequency response and some of these other measurable factors, in ways that can be difficult to precisely define.

Going further, it could also be said that any deviation from the original frequency response is really some form of distortion. But there are many types of distortion, and we really don't have one good measure that can reliably account for all distortion and differentiate between what distortion is or isn't audible in different cases. It ends up being very difficult to define meaningful audibility thresholds for some of these measurements.

But there does seem to be a significant correlation between price and measurements of non-linear distortion. On the other hand, there is very little relationship between price and frequency response, which is still more important.

And even if higher priced headphones do tend to be better on technicalities, it still becomes very difficult to say at what point these differences should be inaudible. It's very easy to find things clearly above audibility thresholds in many headphones < $100, for example. But once you get to ~$300-$500 range, I think it really become debatable. And then when you are comparing Elex vs. Utopia, I have no idea whether there are any measurable differences which could even be audible.

So for now, I would say that if there is a strong consensus amongst listeners who have actually used these headphones, then likely there are differences, whether we have fully figured out how to measure them or not.