Batman's mask only covers half his face. In the orginal Batman movies, I never caught when a stuntman stepped in for the actors playing Batman.
Plus (as I mentioned earlier), the lower half of the face could be CGI'd SLIGHTLY just to help really nail things home that the stuntman is the actor.
Looks at Star Wars: Episode II. Christopher Lee played Count Dooku for real when he was doing non-action scenes and for close-up shots during action scenes where he didn't need to move much.
For the real battles, a stuntman of comparable size was used, and then Lucas had Lee's face CGIed onto the stuntman.
Now if you look REALLY CLOSE, you can tell that the face is CGIed on. But the first time I watched the movie, I couldn't tell (and no one complained about it that I know of, so I guess few people could tell at all).
For Iron Fist wearing a mask, a bare minimum CGI on the bottom half of the face would be needed. And considering most really intense fight scenes wouldn't involve close-ups of the actor's face, the stuntman would work fine. If you want a few close-ups thrown in, movie magic time: just shoot a few scenes of close-ups using the real actor and then edit them into the fight.
BOOM. Done. Handled. Good.