Re: I slowly take off my wizard hat...
Compared to having access to 8th level spells, that's not too unbalanced. As for optimized, continuing to take outsider HD after reaching the minimum required doesn't sound very optimized since you don't get any new abilities. Taking levels in sorcerer or bard instead... Sure, for 15th level character 2nd level spells or 3 level bard's bardic music isn't very good, but provided you choose effects that don't allow for spell resistance you get basically automatic successes.
Actually, if you follow the some of the advancement rules in Savage Species, I think, you can pick new abilities at appropriate levels. While 3 extra HD doesn't provide the same benefit as actual class levels, it does allow to add things such as a 1/day greater planeshift as a logical extension of the greater teleport at will power. Or another spell as a logical extension of charm or suggestion powers 1/day, such as say, Dominate. And lets face it, outsider HD are pretty good with full BAB, all favoured saves and 8+ skills.
Additionally, you can take a spelltouched feat to anchor them to whatever plane they're currently on, removing the weakness of being able to be banished.
Plus, for fluff reasons, I don't like outsiders taking class levels without special circumstances, since they're made for a purpose rather than trained, which should involve a certain level of difficulty in learning skills outside their intended field.
Oh, and when i say PC stats, I mean 32 point buy. Elite Array is minion stats.