It already is presented at 4:3. Snyder wanted to preserve the IMAX aspect ratio.
On the whole, I thought it would have made a better mini-series, not because I don't like longer movies, but because it seemed to be paced as though it were building toward several intermediate mini-climaxes. It felt more like binge-watching than movie watching.
It was definitely a more coherent story than the Whedon version and one that served the characters better.
Grayscale seems gimmicky, but I can see where it could have been part of the original vision that Snyder couldn't get past the studio until he had received strong backing from the fans (as he has gotten over the last few years.) It seems unnecessarily "Lucas-y" to keep futzing with a film once he's had the opportunity to restore it to his original vision, but if he feels it's an important artistic statement, he's welcome to make it. I doubt I'll watch it, though.