Since the original thread on this topic has had no activity in two years, it was suggested that I start a new one.
First off, I confess to being lazy about cleaning the carbon buildup off my 2014 E's exhaust outlets. Had a lot of stuff going on. That's my story, and I'm sticking to it. My outlets, of course, are the trapezoidal shape that exit through the lower bumper. I've hit them lightly a couple of times after car washes. But that stuff bakes on like lava rock, so I never made much progress. Today I decided to tackle them, and I had to go through multiple steps to cut through the buildup.
I first tried straight ammonia, becuase that seems to be the primary ingredient in most metal polishes. That didn't take me far, so I broke out my trusty Meguires Gold Class All Metal Restorer. That took off a good bit of the crud, but not enough fast enough. I then grabbed a non-scratch kitchen scrubber that I had trashed the night before. That also wasn't very effective (I think it was just too worn out). So I tried some #0000 steel wool and a synthetic fine sanding pad. Lubed both with water, and saw a bit more progress. I kept using the sanding pad until most of the crud was gone. In hindsight, I shouldn't have done that, because it or the steel wool left some very fine scratches in the chrome. Unfortunately, they aren't visible in the photos. I did my best to clean that up with some Turtle Wax red rubbing compound, followed by some Turtle Wax white polishing compound. Last step was to finish it off with more of the Meguires.
The final verdict is that they at least look presentable again, although I'm gonna have to go back with additional rubbing and polishing compound treatments to get rid of more of the fine scratches. I'm thinking if I'm diligent about using the Meguires after every car wash, along with a relatively new non-scratch kitchen scrubber, I can prevent the heavy carbon buildup that I fought with today. Hope this helps other E owners.