I would not call the Qs8's overkill, they are awesome and will do fine in that room. Many people use them in smaller rooms than yours.
The EP500 will rock your world, maybe lift your room off of its foundation.
I owned an EP500 for about a year in a much larger room than yours, and it was awesome. I now own the EP600 and am equally enjoying that monster.
The footprint of the EP600 is about the same if you get the vertical model, however, my horizontal 600 takes up lots of room since it is laying down.
It really is up to you, however, I would not be concerned about lack of performance with the 500, it is unbelievable, and for your roomsize it will be awesome.