Hmmm..DC 25 might be a tad brutal for a 10-HD creature.
I'm open-minded about the Spot bonus. A gibbering mouther has more eyes than a beholder, and still only has the +4 bonus from all-around vision. Ditto for the overseer.
If we go for Ability Focus, we might think about boosting the CR. If everybody in a party falls asleep, it could be quite deadly. The problem is figuring out another good feat for an immobile monster. Weapon Focus (roots)?