Does Tenebrous Spell improve shadow spells?

Question

Tenebrous Spell grants a +1 bonus to DC when casting a spell in darkness or dim light conditions. Then it goes on to say (emphasis mine)

A tenebrous spell uses up a spell Slot one level higher than the spell’s actual level, except in the case of spells with the darkness or shadow descriptor or of the illusion (shadow) subschool; this feat does not change the effective level of those spells (but still counts as using a metamagic feat for all other purposes).

The way I interpret this:

• When you apply this metamagic while standing in dim light or darkness, you get a +1 to Spell DC, regardless of the school of magic or spell discriptor.
• If the spell that you apply this metamagic to is of the Illusion (Shadow) subschool or has a [darkness] or [shadow] discriptor, you gain the additional benefit of not consuming a higher spell Slot.

But it was hinted to me that because the spell Slot is not increased for shadow spells, it also doesn't gain its spell DC bonus. This suggests that the increase in DC is merely because you are applying metamagic to your spell, increasing the Slot and spell level. Not because the metamagic itself gives you a spell DC bonus.

If that suggestion is true, however, would that also mean that if I cast a non-'shadowy' spell in the right lighting conditions, it would gain a +2 to DC instead (+1 from Tenebrous Spell in the right conditions, +1 from the increased spell level)?

Considering the metamagic spell's name, I think my initial assumption was right, but can anyone clarify what's the case?

Answer 1

The feat Tenebrous Spell improves shadow spells according to your initial assumption

That's because, according to Metamagic Feats, Effects of Metamagic Feats on a Spell says that

In all ways, a metamagic spell operates at its original spell level, even though it is prepared and cast using a higher-level spell slot. Saving throw modifications are not changed unless stated otherwise in the feat description.

So the feat's benefits don't also increase the spell's actual level (hence the saving throw DC) because the feat doesn't say that happens, but the feat's benefit increases the save DC independent of the metamagic spell's new level if conditions are met.

Ignoring this rule makes metamagic feats in general more powerful. That is, if all metamagic feats increase the spell's actual spell level instead of merely increasing the spell slot the spell occupies, modifying a spell with, for example, the feat Quicken Spell, in addition to changing the spell's casting time, would have the glorious side effect of also increasing the spell's saving throw DC by +4. And, of course, under such rules, most wouldn't bother anymore with the feat Heighten Spell, which is the feat that for-reals changes the spell's level, usually to keep low-levels spells relevant by increasing their saving throw DCs.