Seems to me that any 'fix' that lowers fuel economy would have a real and measurable damage on the owners in the way of increased fuel costs. Wouldn't this just be deflecting the problem from one area to another?
Besides fuel economy heavily depends on load, driving style, trip length, conditions, where you are driving (city, autobahn, ...), how fast you are driving, which tires you're using and so on.
Even if there is a measurable change I think it would still be difficult to convince a court that this is a damage, a customer deserves to be compensated for.
It's be cheaper than a refund (which is unlikely) and would allow them to bring the cars back into compliance.
https://hyundaimpginfo.com/customerinfo/compensation-informa...