No, it's not at all over-kill. It looks like you will have a very nice room. You may want to move the couch in the centre a little closer to the front to enjoy a better soundstage from the 80s. Try to form an equilateral triangle between you and the speakers.
If you are interested in good bass at a small sweet area, get the 600. If you want more even bass around the room, get two or more 500s.
You may want to consider adding a raised floor and putting the couches that are against the wall on this raised floor so that you can have elevated theatre seating.
