The experts will be here soon but I think the answer to your question has more to do about 'what do you want to hear' verses the size of your room. Adding a center and surrounds is not really about adding volume, it's about adding surround effects and center clarity.
So if we take the space out of the equation for a moment,
Re. the center
Do you feel like you'd like to hear the dialog a bit louder or clearer over the rest of the movie/music sounds?
If you are seated in the center, you are already probably getting a good 'phantom' center effect from the to M60s but perhaps those seated off to the sides are hearing the dialog more from the side speakers and find it distracting.
If the answer is "Yes" to any of the baove, then you will enjoy having a center speaker.
Re. the surrounds.
Quit simply, do you want to hear surround effects. Noises coming from beside or behind you. Helicopters flying by, bullets wooshing past your head, etc... If so, then any sized room will benefit from having surround speakers to listen to 5.1 movies and mutli-channel music.
The M60s are gorgeous with music but a sub certainly adds some nice deep depth. For movies, it adds more fun factor than even surrounds for my own self.
However, perhaps I misread. Maybe you do already have a center and surrounds, just not the models you are considering upgrading to. If that's the case, I defer to the experts but can tell you my HT room is slightly smaller and I have M60s, VP150, QS8s and an EP500. None of these 'overpower' the room unless I make them do it. They all sound good at low or moderate volume levels so they can scale down to my room size and still sound good.
The sub might be the only thing to be careful with. It's easy to rattle your neighbors dishes with a big sub even at low volumes, simply because low frequencies love to travel through walls and visit the neighbors. However, if you set it at reasonable levels via its adjustments or via those on your amp, you should be able to keep from getting evicted as still enjoy the dramatic difference it makes.
