My take on it:
these run the hottest when under heavy load, and NOx production is at max level under these conditions.
The highway cruising at 65-75 is easier on the engine, as all you have to do is to maintain momentum by overcoming the rolling and aero resistance.
As long as fix is at its most aggressive when engines are under heavy loads - say climbing long grades, or driving at 85+ mph, folx that do mostly hwy cruising should see very little impact on avg mpgs. methinks
Mine barely consumes any DEF and even if I will have to refill it twice as frequently, no big deal at all.
Most of my driving is hwy cruising under 75 mph. If I loose 2-3 mpgs (my summer time avg is about 50mpg), well, I can live with that. If it is more like 10 mpg loss, combined with noticeable loss in power/torque, then there's always the buyback route.
Remember that settlement requires full disclosure of fix's impact on mpg etc
And I'd image we'd have folx reporting their post-fix #s, and quite possibly, cars will get dyno'd, post-fix, by indys as well