I don't think those shops mentioned can make the headlights looks exactly like the original gunmetal plated finishing, but will get a layer of dull flat black color effect. I have seen the one done by Best One on EK headlights and honestly it's nothing close to the original.
One of my friend did it on his EK by using only clear coat mixed with a few drops of black paint and the end result is excellent, looks excactly like the original EK9 since the clear coat didn't cover up the chrome plated finishing. Once again, I don't know how long it will last since headlights are pretty hot after 1 hour usage. However, he had done that for a year so far so good.
If you really wanna do it, better make sure the guy do a good job. Otherwise, not only you can't get the smoked headlights effect, but a fogged up headlights instead. Or worst you will eventually get a pair of fucked up headlights...