Probably not a recommendation for you, but I'm posting it in case anybody else passing finds it interesting:
I got a 6'' reader rather than 5'' to have an option to read b&w comics in addition to novels, and tested it with a file I built from collected pages of
http://www.diggercomic.com/ - as described upthread: zipped up, renamed to cbz, converted in calibre to PDF.
The PRS-650 turns the pages as quickly as a novel page. And it does have an SD card slot. And the text of that comic is big enough for me to read without zooming. (If only just, but the text seems relatively small when compared to manga I've read.)
But if you have the budget and
mostly want to read comics/manga, something big enough to display a double page spread in one go would make sense.