Just going for Ultima and not sure I use the unit that uses it. When I get Ultima, Exdeath might be 6* and well I rather keep 4x30% MAG than 3x30% MAG + Ultima. Oh well, I still do one ultima.
Assuming only 5 star units
Exdeath with 4 MAG+30 : 363 MAG
Exdeath with 3 MAG+30 and Ultima: 330 MAG
Kefka with 3 MAG+30 and Ultima: 391 MAG
Kefka with 2 MAG+30 and Ultima (slot 4 empty for dualcast) : 357MAG
(data using the best MAG bonus equip available and Ramuh with all MAG bonuses)
Magic damage formula is:
(Unit MAG2 / Enemy SPR) * (1 - Elemental Resistance) * Skill Modifier * Level Correction
Skill Modifier: % of magic damage of skill
Level correction: 10 + (unit lvl/10) = 10 + (80/10) = 18
*Lets assume that enemy has 100 SPR
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So first Exdeath with 4 MAG+30 cast a meteor:
Meteor: 250% neutral magic damage, ignore 25% of SPR
Meteor Damage = 300.433
Now, Exdeath with 3 MAG+30 and Ultima
Ultima: 280% neutral magic damage, ignore 25% SPR
Ultima Damage = 278.087
Now, Kefka with 3 MAG+30 and Ultima:
Ultima Damage = 390.396
Now, Kefka with 2 MAG+30 and Ultima:
Ultima Damage = 325.453*2 (for DualCast) = 650.906
So yeah. A 5* Kefka with 2 MAG+30 and Dualcast is theorically more powefull than a Exdeath 5*, even with ultima equipped.
Edit: For your interest, a 6* Exdeath does 371.612 (743.224 with W-Magic) Meteor damage (4 MAG+30), and 327.995 (655.990 with W-Magic) Ultima Damage (3 MAG+30 and Ultima)
Edit2: For the Lolz, a Kefka with 3 MAG+30 and dual weild (rod of gravity and Ice rod (417 MAG (!!!!)) does 364.749 (729.498 for second time) of hyperdrive damage.