Similarly particle filters, catalytic converters and whatnot reduce efficiency via exhaust backpressure.
So all in all, reducing non-CO2 emissions do come at a cost in CO2 emissions (or fuel consumed, if you like). Is it as much as 15%? No idea.. And is it all worth it? I'd argue yes, old-school diesel exhaust is nasty stuff.
As for your later point...I very much concur. I started masking (n95) during the pandemic, and haven't stopped. I have a... large number of health issues including several respiratory ones. Exhaust in general really is nasty stuff. I live in pretty quiet smallish (100k) town, and it can be bad enough around here with all the pollen, but I wasnt recently on a whirlwind trip through the north east that saw me visit the dense urban cores of DC, Philly, Manhattan, and Boston. The difference in odor on the occasions I'd take my mask off on the sidewalk were kinda shocking as someone not used to it.
The "I bought a tuner" or "I bought a kit" guys don't care or know enough to care.
Once you start talking about people who are reading turbo specs and whatnot people do very much care (because turbos and injectors are expensive and you want them both to be compatible so they get the most out of each other).
(Emissions control, reduces NOx in diesel exhaust.)
