Let's not forget what we can do unassociated class levels. Take e.g. Choker (CR 2, 3 HD), add 18 levels of Cleric, you have a CR 11 monster with the ability to throw two 9th level spells per turn.
I am fairly sure that non-associated class lvs apply only up to the monster's base racial HD. So for a choker with 3 HD, only the first 3 cleric lvs would be treated as unassociated, the rest would be associated (and thus improve its cr at a 1:1 ratio). A choker cleric18 would be cr19 (base2, +2 from 1st 3 cleric lvs, +15 from the remaining 15).
The real abuse would come from combining spellcasting classes with monsters that have a lot of racial HD for their cr. For instance, a storm giant sorc20 would be cr24 (and have caster lv24 after factoring in practiced spellcaster). Walks over a vanilla human sorc24 npc any time. An elder elemental cleric24 would be cr23...
Thankfully, they are simply guidelines.
But you are right in that with its racial wis bonus, it makes for an excellent divine spellcaster. Ur-priest anyone?
-The multiple action abuse seems fun, being able to overcome the action disadvantage while still keeping things to a single monster is attractive. Does anyone know of any other monsters that has this ability? I know of the Choker, Thoon Elder Brain, Chronotyrin and have just learnt of the Sharn...
Nagahydra (serpent kingdoms), cr18, casts as sorc15, can cast up to 7 spells each round (1 per head).
Nilshai (unapproachable east), cr8?, casts as sorc8, also gets an extra standard action each round (I think).
The aspect of demogorgon (online web enhancement) gets 2 full rounds worth of actions each round, but has no spellcasting (so it is mostly just melee attacks).
Beholder can effectively get extra attacks if you interpret that if it hovers directly above the party and split them into 4 quadrants, then it can fire all 10 eye rays (assuming you aim no more than 3 at each quadrant).
You may also want to ensure that your bosses are able to take full advantage of the action economy (ie: make full use of move, standard, swift and interrupt actions where possible, leaving as little to waste as possible).