My current character is an Illusionist. And while I feel it's definitely a decent spellschool, it's not as powerful as Enchantment if it's just technical power we're talking about.
The Phantasmal Killer's usefulness is heavily reduced by the dual Will and Fortitude saves. It has a few pros though, like the one Verk mentioned (which is also kind of a minus since it lowers the DC), and the fact it's a medium ranged death spell that doesn't require a touch attack.
Color Spray is a powerful mind spell for being first tier, but its relatively short range and the cone attack makes it more difficult to use than most other mind spells.
I'd say Charm Person, Blindness/Deafness, Hold Person, and Confusion outweigh Color Spray and Phantasmal Killer by far. But then again, it's not like everything can ever be perfectly fair and equal and it's all about the RP anyway.
If focusing in Illusion spell school was to actually get a boost, my suggestion would be for it to enhance the invisibility spell somehow, like counting as a few levels higher, as was done with Abjuration spell school. Would make sense for an illusionist to have better grasp of this backbone of their spell school. Or boost the Shadow Conjuration spells. No one ever uses those (and for good reasons). I guess they'll get a bonus just from Summon Shadow being looked into though.
And I'm not posting this because I want a boost for my character. He'll most likely be dead before any of this could happen anyway!
Edit: Of course, being a proper Enchanter, i.e. having it as chosen spell school, not just the feats, would mean losing Illusion spell school. And losing the spells Invisbility, Improved Invisibility, Ghostly Visage is definitely a bigger hit than losing the Enchantment spells as an Illusionist. When I wrote my post, I only had the Spell Focus feats in mind.
