[3.5] How balanced are Thri-Keen?

[NEO|Phyte]

Black Ethergaunt: LA +4, +20 Int, and ability to cast spells like a 17th level wizard, not to mention a whole set of other broken abilities. 9th level spells at 5th level, anyone?

I suspect you're forgetting the racial hit dice. You don't get to ignore those.