The thing is, chickens don't know from math, what they know is 'happy' (or at least 'not overly stressed').
I would say that while adding a loft would not *hurt*, it is not going to be the same as if you added an equal amount of floorspace. (I think it would be a mistake to make a 6x6 loft, btw, as you will be unable to adequately clean it or reach to the back wall to retrieve things unless you build the *heck* out of it. It is hard to reach more than 3' deep, 4' *tops*)
As another poster said, the first thing to make sure is that you have enough length of roost -- you need at least 8-10" per chicken and that's if they're all in a getting along mood.
I am assuming this is just your coop space and you have ample (like, at least twice as much) outdoor run space as well? If so, it is not an egregiously small coop, it's about the same sq footage per chicken that most people here have. It may not be doing your birds any great favors but they will probably survive. If you run into problems (picking, etc) you might have to rethink things, but I don't see that there is an enormous Space Emergency. Again, assuming there is additional ample run space.
Good luck,
Pat