In my experience, a mid optimized wizard will end the game with an intelligence of around 30, so by 20th level, they are casting 5 ninth level spells a day. The black ethergaunt with an LA of +4, would be casting 4. The wizard would be slightly more likely to overcome spell resistance, and have slightly larger range on his spells, however the ethergaunt would have +10 to all spell save DCs, which is the equivalent of getting to apply a +4 metamagic feat to every spell you cast for free.

That's before factoring in all of the Black Ethergaunts other abilities, some of which I would argue are equivilent the of ninth level spells (spell immunity and enslave).

I would argue that the Ethergaunt is too powerful to have 9th level spells by LV 20. I vote LA +5 for the Black Ethergaunt.

Now you could argue that a Wizard can polymorph any object himself into a Black Ethergaunt, then Persistent Shapechange into an Ethergaunt, and functionally be a Black Ethergaunt. Therefore the monster in question deserves a very low LA, perhaps just a +1 or +2. Not once in an actual game have I seen this tactic actually used, so for now, I'll just ignore it.