It would only lower the fuel economy (as advertised), if they didn't apply the cheat during those tests. If they did, the cars fuel economy right now is higher

It would only lower the fuel economy (as advertised), if they didn't apply the cheat during those tests. If they did, the cars fuel economy right now is higher than advertised. 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.