Yes ate too much is just that, too much. You ate so much is that you ate a lot, more than normal but it wasn't too much.

You eat too much would be the correct because Too and so are both intensifier. Too means more than needed of something and so makes the adverb or the adjective stronger. Example: I ate too much. (I ate more than needed) I ate so much. (Can eat much more if you want to)