“...how much mileage, longevity and performance will I lose?”
Short answer, just about all of it.
But think of it, by making all the VW Diesel engines fully compliant with the existing law, you will be “helping” the environment. Also, those greedy capitalists that are in charge of VW worldwide operations will be suitable humbled and scorned, for allowing this “travesty”.
Can’t say much for the resale value of these “remodified” VW Diesels, though.
While I agree with the sentiment, VW acted stupidly and betrayed its customers with this debacle. Even if the rules suck, one doesn't throw its customers under the bus because it wanted to inflate performance an economy specs. They even had commercials with the old ladies saying how diesels didn't have the get-up-and-go, them having a riot with the unexpected performance.
I call that fraud.