Someone who's gone to the trouble of modifying their vehicle in that manner may be beyond the point of reasonable discussion. On the other hand, that's likely to be a pretty small proportion of people, overall.
The unreliability of the emissions gear is usually what comes up when I talk to diesel folks. Rolling coal is a fun extra for them.
Just musing now, but the diesel guys today seem to say a lot of the same things the gas guys said in the 80s with their EGRs and cats and vacuum lines galore under the hood destroying their reliability. Gas got variable timing and direct injection, and is no longer so fragile. Maybe there's some room left for innovation there, maybe not.
Yes, you need to keep it clean, drag affects all things equally, and dirty anything means more drag.