If I'm a 15th level Wizard, capable of casting 8th level spells, can I use Spell Perfection and Sacred Geometry to apply meta magic feats to a total effective spell level of 9th?
Benefit: Pick one spell which you have the ability to cast. Whenever you cast that spell you may apply any one metamagic feat you have to that spell without affecting its level or casting time, as long as the total modified level of the spell does not use a spell slot above 9th level. In addition, if you have other feats which allow you to apply a set numerical bonus to any aspect of this spell (such as Spell Focus, Spell Penetration, Weapon Focus [ray], and so on), double the bonus granted by that feat when applied to this spell.
Benefit(s): When you take this feat, select two metamagic feats you do not yet have. When casting a spell, you can perform the steps below to spontaneously apply the effects of either or both of these metamagic feats, as well as any other metamagic feats you have, to the spell without expending a higher-level spell slot. Using a feat in this way increases the spell’s casting time to the casting time it would take if the character were a sorcerer or bard (sorcerers and bards using this ability increase the spell’s casting time by two categories); applying the Quicken Spell metamagic effect negates the increased casting time but still contributes to the modified spell’s effective spell level. You can take this feat more than once; each time, select two additional metamagic feats, adding their effects to the list of possible effects you can apply to spells with this ability.
When casting a spell using Sacred Geometry, first determine the effective spell level of the modified spell you are attempting to cast (calculated as normal for a spell modified by metamagic feats). You can apply any number of metamagic effects to a single spell, provided you are able to cast spells of the modified spell’s effective spell level.
Refer to the Prime Constants table to determine the prime constants that can be used to cast a spell of the desired effective spell level. Then roll a number of d6s equal to the number of ranks you possess in Knowledge (engineering). Perform some combination of addition, subtraction, multiplication, and division upon the numbers rolled that gives rise to one of the relevant prime constants. If you can produce one of the relevant prime constants, the spell takes effect with the declared metamagic effects, and you expend a spell slot of the unaltered spell’s level. If you are unsuccessful, you fail to cast the spell, the action used to cast the spell is lost, and the spell slot is used up. The DC of any concentration check to cast a spell affected by this feat uses the effective spell level used to determine the prime constants, even though a successful casting of the spell does not expend a higher-level spell slot.
Sacred Geometry no. Spell Perfection yes.
You can apply any number of metamagic effects to a single spell, provided you are able to cast spells of the modified spell’s effective spell level.
You can't cast 9th level spells at 15th level so the most you can use metamagic to boost is upping your lower level spells to 'effectively' 8th level(although still using the lower level slots because Sacred Geometry is disgustingly overpowered).
Spell Perfection doesn't care though, so you could Quicken a 5th level spell. Keep in mind, metamagic feats you pick with Sacred Geometry you don't actually have and can't use them with Spell Perfection. But you'll probably take all the good metamagic feats anyways.
I understand both individually, but what if we use them in random? Could I use Sacred Geometry 1st and then add on a +1 meta with Spell Perfection?
I would say yes. Lets say you have fireball as perfected spell. You use sacred geometry to add silent and still to it making it 5th level spell. Then you use Spell perfection to add Quicken. So you have an effective 9th level spell out of a 3rd level slot.
You could use Sacred geometry to add Dazing and Empower to fireball making it 8th level spell since as a 15th level wizard you can cast 8th level spells. Then you can add Intensify to the spell with Spell Perfection.
Hopefully someone can prove me wrong since Sacred Geometry is an abomination that should be purged.
If your GM is going to let you use SG then you can get away with whatever you like. Don't sweat it.