Ha, actually this is a biggie for accents I think. If you ever watch the Doom film - and I am not advising it, despite it having Rosamund Pike, Karl Urban and The Rock in it - you’ll note that Rosamund Pike spends a lot of time in situations where she clearly should be shouting but just sort of quietly grinds out her lines and I’m convinced it’s because her American accent will go to pieces if she yells.
I’m sure she must yell a bit in Gone Girl, though, so maybe it was just weird direction or she got a much better coach for a big time film.
Still, doing an accent and yelling is meant to be tough so if they’ve got Elba having to be quite loud I can imagine his accent is dropping. (Also, I mean The Wire was a very serious business TV series where I’d guess there were a lot of people helping him to keep in line.)