I didn't see this in the responses so far, but I may have missed someone say this:
Since this is simply an indication of what will the the priority of the developers in the base game, this is fantastic news. Consider that, in CK2, the Devs focused on Feudal Europe. They then went back and focused on the Islamic states. Now they're going back and focusing on the Byzantine Empire. By staying focused on one area at a time, rather than just trying to make things good overall, I think that CK2 is developing into a much more complex and varied game than it would have been if they just tried to cram everything in vanilla.
So, what I'm saying is that its quite likely that we'll get DLCs that expand things for the lower tier nations. And I'm quite happy.
Though yes, if the Byzantines are Tier 3 (good enough for me), then USA should be tier 3, too. They're kinda mirror images of each other, within the scope of the game (one, a great power before the scope of the game reduced to an insignificant backwater, the other, an insiginificant backwater about to become a great power after the scope of the game).