Well, quality of life may be better. Then again, quality of life during humanity's expansionary phase was also probably much higher. The Imperium doesn't have draconian laws and oppressive policies because the High Lords of Terra are complete monsters; The Imperium is the way it is Because that's the way it needs to be to survive at this point. For a species as psychically attuned as Humanity is, Warp corruption is a major, major concern. As the Tau expand and absorb more species that don't have the luxury of a small presence in the warp, they're going to have to become more like the Imperium to prevent their own Horus Heresy from taking place.

And then there's the issue of FTL; The Tau's empire only works because it's tiny compared to the Imperium; Without FTL comparable to the Imperium's (Only possible for them with Navigators and Astropaths of their own. Which again, requires Psykers; And that involves courting disaster), The Tau's responses will be too sluggish to deal with threats.