Two questions:
1) Does Spell Perfection allow a Partial Caster (an Enigma Mesmerist, to be precise) to add, for example, Quicken Spell Metamagic to a 3rd grade spell (turning it into the equivalent of a 7th grade spell, which a Mesmerist usually can't cast)? My GM thinks not, but that's because he thinks that RAW doesn't allow it. If I can convince him that it's fine by RAW, he'd probably allow it. If there was ever a developer comment on the matter, it would be ideal.
2) If I can't convince him and am limited to metamagic that I could still theoretically cast, would Spell Perfection still be worth it? (I'm planning on putting it on Shadow Enchantment.)
1) RAW is unambiguous: You can apply it "as long as the total modified level of the spell does not use a spell slot above 9th level." 9th level, not "the highest spell level you can cast", and there is nothing at all to suggest it should work differently for a class that can't cast 9th level spells.
2) My first instinct is no, since Enchantment is famous for being ineffective against broad groups of enemies, and having a large amount of spells that all do basically the same thing (Mind-Affecting Will Save-Or-Lose). Even if you have some ability to make mind-affecting work on creature types that are normally immune, the spell itself says that those creatures automatically pass their Will saves anyways. I don't know much about the Occult classes so I can't suggest a better spell off the top of my head.