Did he? I stopped watching the show in the early part of season 3 so I can't say what's happened since then. Seen a few clips, watched a Crisis episode or two, but that's about it.
If the character developed into a more recognizable Barry, then great. And just like Miller, I don't have a problem with Gustin's performance, they both did fine with what they were given, I just didn't think what they were given resembled Barry too closely. But if later seasons fixed that for Gustin, then I'm glad. And supposedly Miller gets a character arc in his film where Barry grows up and becomes a little less neurotic. Which would be great too, if the actor wasn't such a wreck and likely to be replaced, along with his version of the character.