The other thing that people don't talk about enough is that diesel is not alone in increasingly complex emissions systems. These benefits you talk about, increased power and MPG in gasoline, come from the switch to direct injection - GDI. However:
"
GDI enables better fuel economy and therefore a further reduction in CO2 emissions compared to fuel port injection engines
GDI engines show significantly higher PM and PN emissions while compared to fuel port injection engines"
-
http://energy.gov/sites/prod/files/2014/03/f8/deer12_bischof.pdf
EURO and the EPA have caught on to this and are requiring particulate filters to be installed in GDI engines as well. Plus, GDI engines
emit higher CO2 levels, requiring more expensive and complicated catalysts.
If diesel is done, gasoline isn't far off either. IMO all this crap misses the point. We're trying to fix the symptom, not the problem - the problem is that we drive too much. We commute too far, drive too far to go shopping, have pathetic public transportation systems etc. Plus, since there are no regulations on what we are ALLOWED to drive (I can take a 1 ton truck rather than my Jetta TDI, fuel economy be damned, whenever I please) all these regulations serve to do is unfairly harm compact economy car drivers with overly-expensive, complicated, hard to repair emissions equipment.