After Effects is used and has been used on many, many feature films.
I use After Effects to generate material for cinema regularly. Just make sure the technical requirements of the bureau who does the film transfer are adhered to strictly.
Is it possible to create comps in After Effects in the 2K or 4K size and edit them in Premiere, to be output to film later?
Yes, perfectly. However, for that to be halfway painless, AE ideally needs to run on a 64bit operating system so it can take full advantage of the maximum addressable RAM (4GB on Windows, 3GB on Mac). And of course you will need a lot of processor power as well. The question also is, what you are going to output - true 2K/ 4K or just 24fps HD 1080. The latter is quite common to use for short films and commercials in cinemas these days. As for other tools - well, depends on what you do. Fusion and Nuke may offer more robust features in some areas, but e.g. if you work based on Flash files, they may not be all to helpful for your project since the won't handle SWFs natively. Likewise, going back and forth between Premiere and the comp app would not be as straightforward. It's really a mixed bag, but you should consider that these programs come from the world of movies where a per-shot based compositing approach is used and other features are predominant. This may not lend itself to what you plan on doing. also think about the day when you may need to get other people on board with regards whzat tools they use and how you exchange data as well as their specific skills. You may even find that your best buddy who so far has only been using Photoshop to draw background plates has a full Master collection and would be able to use AE and Premiere as well without hassle, so rather than shelving out more money for otehr tools, that may already swing the decision in a specific direction....