If:
Greater quicken metamagic rod 170,000 gp
DESCRIPTION
The wielder can cast up to three spells per day that are quickened as though using the Quicken Spell feat.
And the feat is : Level Increase: +4 (a quickened spell uses up a spell slot four levels higher than the spell’s actual level.)
The feat can never alter a spell above 6th level, so can the rod quicken 7-9th level spells?
Or is that limitation applied as it modifies 'as the feat', or is that to strict an interpretation?
The feat can never alter a spell above 6th level, so can the rod quicken 7-9th level spells?
Yes, the rod can quicken even a 9th-level spell. The limitation of which spell levels a metamagic rod can affect are part of the item itself, i.e. being either a Lesser, Normal, or Greater Metamagic Rod.
Otherwise, the very existence of a Greater Quicken Metamagic Rod wouldn't make any sense.
If:
Greater quicken metamagic rod 170,000 gp
DESCRIPTIONThe wielder can cast up to three spells per day that are quickened as though using the Quicken Spell feat.
And the feat is : Level Increase: +4 (a quickened spell uses up a spell slot four levels higher than the spell’s actual level.)
The feat can never alter a spell above 6th level, so can the rod quicken 7-9th level spells?
Or is that limitation applied as it modifies 'as the feat', or is that to strict an interpretation?
So...
Nothing in the general rules of metamagic, nor in any of the feats, prevents you from altering a spell to the point of being above 9th level. What stops you is your inability to ever cast such a spell in standard play, since you don't have 10th level (or higher) slots.
Any Metamagic Rod effectively reduces the cost of its metamagic to +0, at the cost of a daily charge, and regardless of whether you have the feat.
Metamagic rods hold the essence of a metamagic feat, allowing the user to apply metamagic effects to spells (but not spell-like abilities) as they are cast. This does not change the spell slot of the altered spell. All the rods described here are use-activated (but casting spells in a threatened area still draws an attack of opportunity). A caster may only use one metamagic rod on any given spell, but it is permissible to combine a rod with metamagic feats possessed by the rod’s wielder. In this case, only the feats possessed by the wielder adjust the spell slot of the spell being cast.
Actually, Spell Perfection does prevent you from being able to reduce the cost on a metamagic when the metamagic increases without reduction goes above what your max level spell castable is, but that's the only limitation.