I have looked at quite a few different methods to process comic files (.cbr & cbz) for reading on ebook devices and been unsatisfied with the results. I recently upgraded my copy of
ComicRack and discovered that in addition to being able to convert into .pdf ComicRack also allows you split double pages, adjust resolution, and make changes to contrast & brightness. It doesn't convert from color to greyscale, but with everything else that it does it might be worth a look.