Sosin, it really depends on the technical reasons that VW chose to include the cheat software on the EA288 US models. If the actual SCR catalyst that is used is undersized and the fix involves changing DPF/SCR catalyst, then yes you are right that the engine characteristics will change (flowrate, backpressure, etc) and that can have an effect on fuel economy and driving characteristics.
If the technical reason is that VW wanted to reduce adblue dosing to align with 10,000 mi service intervals, then the impact to fuel economy and performance will probably be negligible. Currently, court docs seem to support this, but we won't know until the fix is officially approved. The additional NOx sensor that is talked about is to help the SCR system better report NOx conversion efficiency to the ECU so that the system can do a better job at controlling NOx emissions. Hard to tell if DEF dosing needs to increase under a heavy load if there's no sensor to report the NOx coming out the end of the tailpipe.
Anyways, back to the question at hand, I have a 2015 TDI Golf, which I love, love, love. I have had zero problems with it at at 35,000mi. Basically the buyback give me the MSRP of the car. In theory I want to keep it, but in 2 years, my life situation will likely be different. I will be married, probably have a kid on the way, and be looking for a new vehicle with a little more cargo space. A TSI Golf Sportwagen perfectly fits the bill, except that I would obviously love a TDI. So I may do the buyback only because it is the best deal I'm going to get on the car, right when I will be interested in a vehicle that better fits my life situation. (Also my lovely fiancee can't drive stick, and is resistant to learning, but that's a whole different story...lol. I'm not the first forum member to fight that battle...
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So I'm gonna wait it out and see what happens. Lots of unknowns right now. Will a fix ever be available? Will TDIs come back in 2018, 2019? We just don't know enough info yet. We've got two years to make the decision. And for those that argue about the mileage component, with the way I drive, I could potentially lose 2K value on my buyback if I do it in 2017. Well, that's still less than the depreciation that I normally would have experienced over a 2 year period, so it doesn't bother me in the slightest.