For this movie Director's Cut will definitely change my mind. There are very important scenes left out. I don't think that Spider-Man 3 is a bad movie; there are a lot of worst sequels that are coming out. But I think that Spider Man 3 was badly edited. I can't swallow the fact that they deleted that beautiful scene with Sandman and his daughter with the sandcastle, that single scene would have made Sandman's character return to the beginning of the film (when he is looking at his daughter, which is exceptionally emotional and rich scene) and rounded his character as a whole. But they decided to that atrocity of a scene in the alley when Sandman meets Venom. In my opinion, that is the worst scene in the film. I also think that Norman was somehow not that much present as we all expected him to be. So every other deleted scene with Norman and Harry's descent into madness would be great. Every other scene with Eddie and Gwen will also contribute to their characters.
As Spider-Man 4 is underway, and there is even talk of part 5 and 6, I think that an extended version will come eventually, for two most obvious reasons:
One.. that this film was meant to have that extended edition, they wanted to see how Spider-Man 2.1 will do, and I think that they intentionally cut this film to later have this longer version.
And second.. that there will be a big gap between Spider-Man 3 and Spider-Man 4 and that the audience must be remembered and returned to the world of Spider-Man, and what better way to do that than with a completely different and longer cut of the film.
(I just hope that the Sandman/Venom scene will be replaced by that alternate scene with the sandcastle, and if we are to expect a totally different, new cut of the film, that will deal more with Flint's character and his family then I believe that scene will find it's way there)