I am not a fan of Smith at all...but I think the biggest problem is that they kept his Will Smith face when he is in 'Blue Genie' form. It would have been much better if he looked more cartoon-y as the 'Blue Genie'. Also from what I've seen Genie has aura around him when he is the 'Blue Genie' (see picture below). The aura gives the perception that he can morph and change into anything at anytime. It also gives the effect that he is other-worldly...it's more magical. Without it, he just looks like a blue floating person. It's pretty off-putting. I don't think any performance Smith can give can change my thoughts on his miscasting. Ideally it would have been nice to see James Monroe Iglehart on the big screen.
EDIT: And maybe they have plans to continue to tweak the visuals until the movie is released, but will they? Who knows.