Ive always played with a particular understanding of how heighten spell works and recently someone brought into question my understanding of it. I would like to point out that this is similar but different from another question I asked.
Heighten Spell (Metamagic)
Benefit: A heightened spell has a higher spell level than normal (up to a maximum of 9th level). Unlike other metamagic feats, Heighten Spell actually increases the effective level of the spell that it modifies. All effects dependent on spell level (such as saving throw DCs and ability to penetrate a lesser globe of invulnerability) are calculated according to the heightened level.
Level Increase: The heightened spell is as difficult to prepare and cast as a spell of its effective level.
When heighten spell is the only metamagic feat applied to a spell, its obvious of how it works, and is stated right there in the feat. So a first level spell prepared with heighten spell as a 3rd level spell IS a 3rd level spell for all purposes (except the spell spell immunity which is something else).
Its what happens when heighten spell and another metamagic feat are used together. So lets take that first level spell again, this time lets apply empower spell which increases the spell level by 2. So to prepare the spell requires a 3rd level spell slot. Now I add heighten spell and use a 5h level spell slot. Here's where the confusion starts. Is the spell treated as a 3rd level spell or a 5th level spell for purposes such as overcoming a lesser globe of invulnerability?
Yes the spell was prepared in a 5th level spell slot, which is 4 levels higher than the spell level, but 2 of those levels were used by empower spell. So does heighten spell only increase the spell level by 2 because thats all that was left, or is the spell slot itself the only thing that matters with heighten spell?
If it matters I always thought the spell was treated as the 5th level, and not the 3rd level.