Couatls are hard to balance. Their nine excellent (as far as RHD go) RHD give them flight, telepathy, ethereal jaunting, shapechanging, improved grab, constrict, Large size, ability bonuses, poison, darkvision, a natural attack, natural armor and Eschew Materials as a bonus feat.
Oh, and they gain spellcasting on top of all that. Sorcerer spellcasting. Sorcerer spellcasting except you can pick your spells from the cleric list too, with a handful of druid spells thrown in on top of that.
Now, anyone who's ever seen the cleric list knows that there are a lot of fun things you can now cherry-pick from it. Death Ward? Holy Word? Maybe go full gish and take Divine Power? It's all possible with this wondrous mind-reading jungle snake.
My standard tactic when dealing with monsters that get very strong abilities is simple: raise their LA to the point that they can't get 9th-level spells/maneuvers. In this case, it'd come down to +3 LA.
Is that too much? I think not. There'd be a substantial exchange of spell power, yes, but in return one gains excellent HD and very useful abilities.
Final verdict: +3 LA.