For example, both Fire and Fira would imbue my Spell Fencer's attacks with fire damage. Does using Fira (a level 3 fire spell) over Fire (a level 1 spell) cause my attacks to deal extra damage, or are they both just now "fire" attacks and the extra MP I used for the more expensive spell is wasted? How much more damage would using Fira give, if any?
Your attacks will do more damage with a higher-level sword magic applied.
I ran a few tests tests of this: the same character with the same equipment and level hit the same same monster with varying levels of sword magic applied. I averaged 3 data points for each sword magic.
- Blizzard: 1216 damage
- Blizara: 1530 damage
- Blizaga: 1931 damage
I then added Sword Magic Amp and exploited the enemy's elemental weakness:
- Fire: 2634 damage
- Fira: 3421 damage
- Firaga: 3702 damage