A fair amount of outsiders above CR10 can be improved just by changing their feats to be not-awful and handing them one of the reach weapons they're proficient with but never seem to be used in default stat blocks.
+1. Hell, with a lot of them, if you just look at their abilities and come up with non-retarded tactics that take advantage of them, they can go from mediocre to brutalizing. Look at the vrock, for example. It can fly, and has telekinesis, greater teleport, and mirror image at will.
But its stat block says:
Despite their advantage in mobility, the vrocks’ deep love of battle frequently leads them into melee combats against heavy odds.
Why the
hell would it ever get into melee? It can fly around with a bunch of mirror images (to foil arrows and spells), "nuke" the crap out of enemies from nearly 1000 feet in the air, and Teleport away if they close the distance somehow. True Seeing won't even help, because the range on that is only 120 feet.
Changing its feats around can help even more, of course, like giving it Mindsight and Quicken SLA (Mirror Image), for example. Add a few cross-class ranks in Use Magic Device, give it some wands, and use that Teleport to escape to heal up and buff between skirmishes, and it has jumped up several CR values for sure.