I take Elemental Focus (fire) to add +1 to the DC for all of my spells that deal fire damage. If I also take Elemental Spell (fire), then cast an elemental lightning bolt, does it receive the DC bonus from Elemental Focus? What if I choose to only change half its damage to fire? Likewise, if I took Elemental Spell (electricity), then cast an elemental fireball to change its damage type to electricity, what happens then?
I would assume that Elemental Focus would apply in all cases, as the specific text states that it applies the bonus to all "spells that deal damage of the energy type you select". Since the hypothetical elemental lightning bolt in this situation would deal fire damage, even if only as half of its total damage, and thus qualify for the feat. Evidence for this lies in flame strike, which is only half fire damage, but still has the "fire" descriptor.
Is there any precedent for this or any more evidence for either side?