IME the 'cleverness' of the AI doesn't improve with difficulty; it just gets more mechanical advantages to play with. The AIs don't react to an emergent threat, i.e., the player, or even a present and growing threat on Normal difficulty any more than they do on Genius.
When playing against the AIs the game is pretty much over once you've got 25% of the galaxy to yourself against more than three remaining AIs. At that point the only danger you face is your own boredom. (Most 4X games fall into this pattern tho.)
When playing against Humans it's not quite as clear cut, mostly because even a seemingly beaten Human can deal a sneaky blow that shakes the game up a bit; however, the advantage of playing against Humans is just as much a bust as it is a boon because the game is so absurdly exploitable that PvP isn't all that enjoyable of an experience.
