I don't believe M80s would necessarily be "overkill." But, if you want to save a little dough, I think the M60s would do just fine in your room. I have the M60s in a room that is 11.5 x 18 x 8, with one side half open to a dining room, and they are more than enough. It is generally thought that the M60s and M80s sound very similar, but the 80s go a little lower in frequency response, and can play louder. My M60s are PLENTY loud enough for me.
"People generally quarrel because they cannot argue." - G. K. Chesterton