My dad was telling us how, given that my spouse drives a lot of highway kilometers to work, we should totally get some air filter. A few days later I randomly realized: why don't we put that filter on the exhaust if it's so bad?!
It would only work if exhaust filters were mandatory for all cars, since the environment is "free" to ruin for anyone it doesn't make economical sense to install an exhaust filter unless you're legally obliged.
Sure, we already have a load of regulations to make cars less pollutive. Would this be unreasonable? People are already putting this in their car, just not on the business end.
It's also not just other drivers or the poor environment: a huge number of people (I'd speculate that it's the majority) live near busy roads. Adverse health effects from breathing near a busy road are common knowledge.
Not just the performance, I think the less efficient combustion (from backpressure?) would also lead to more CO and perhaps worse mileage so more CO2.
I don't know how the numbers stack up though, maybe you could make the filter really big, like a meter wide or something and the airflow would be good enough? The filter would need replacing/cleaning quite often as well, probably every 10,000km or so.
That is essentially what the catalytic converter does. However, it can have a small negative effect on engine efficiency, especially when exhaust backpressure isn't optimal.