So I've been playing around with a 12th level build for a PF game that incorporates the Complete books and Races books, and...ye gods.
Gnome Illusionist 7/Shadowcraft Mage 5
1. SF (Illusion)
1. Extend Spell (flaw)
1. Earth Sense
3. (Metamagic feat of +2 or less)
5. Heighten Spell
6. Earth Spell
9. Arcane Thesis
12. Spell Perfection (silent image)
So: mostly, a very basic shadowcraft mage build (really, a little too basic--nobody would take 7 levels of illusionist--but I want to keep this to bare bones right now and focus on the feat interaction.)
Spell Perfection, though, makes significant changes.
The first effect of Spell Perfection is that you get to apply any one metamagic feat you have to the spell without increasing the level or casting time, as long as the total modified level of the spell does not use a spell above 9th level.
Whoa.
Now, obviously, we'll be using this with Heighten Spell. The upswing is that EVERY spell slot the character has can now be converted into a maxed-out Shadowcraft Silent Image, heightened up to 6th level. (We're going to assume here that the DM isn't insane enough to allow Gnome Illusions with PF cantrip rules, and thus provide an unlimited number of Silent Images per day, so let's go with "Every spell of first level or higher.")
But there's more. Much, much more.
If you have other feats that apply a set numerical bonus to any aspect of the spell, that bonus is doubled.
SF: Illusion? +2 to the save DC. Not bad.
Arcane Thesis? The +2 to the CL of the spell becomes a +4. If your DM is generous, the reduction in metamagic costs may also be doubled to a 2-level reduction, but that's a little more iffy.
Earth Spell? Well, the bonus to caster level isn't really "set," so that probably won't increase...but the +1 to the overall level of the spell IS. Our Heightened Silent Images will now be TWO levels higher.
So: our 12th level SCM can take a first level spell slot and Heighten it to 6th level for free. Earth Spell will then kick in and raise it to an 8th level spell, allowing the mimicry of a 7th level spell that is 100% real, with a +2 to DC and a +9 to CL.
That's...rather good, really.