I don't think you get why I made that swap in the first place, and you missed the greater point. If you're in a certain party, depending on everyone's skill level/flexibility, you do what's best for your group. Triple melee comp and your healers aren't struggling for mana? Bring SMN and use Ifrit. Radiant Shield + Devotion will beat out the BRD crit + Foes buff. Are your Healers struggling for mana in general? Then bring at least one BRD/MCH. Aggro issues on your WHM? Does someone like NIN? Ask if that player enjoys AST. Or ask your tanks what they're doing with Shirk/Tank stances. Hell, ASTs will feed their best DPS all of their single target cards in order to get the best edge when AoE cards don't line up.
Our party ran double caster initially because our party liked RDM/BLM and the RDM was new to XIV raiding. They were a carry over from WoW, and played a different role in that. RDM just made sense for them to play till they got used to XIV's combat. It's only because of our DPS disparity at the time that I swapped from BRD to SMN. Swapping out one of the bottom two DPS in the game to the top DPS that ALSO buffed the raid damage more just made sense within the party. Theoretically, if Composure existed, our RDM would be using that in our group too, for that reason.
If SE cared about all party viability they'd allow for all job combos to be played without Limit break penalties, and limit gear drops/weighting to what your composition brought to the raid/dungeon (which every static would appreciate the HELL out of, I might add), and either homogenize all the raid buffs, or add more options than we have currently to buff the raid in more conditional ways. If SE's not going to fix the NIN/BRD/DRG trinity by patching them directly, then I'd rather they attempt to unseat that trinity by buffing alternative classes/comps, including offering more ways to buff party members. The reason for the phys/magic split at all is to limit the power of a particular raid buff, and the reason for higher magic buff weighting is so when they do apply they can make up for the healer/tank/phys vs caster dps disparity. Hell, I'm for more magic based DPS in other roles too but ideally I'd rather have more freedom over what our groups are allowed to bring.
All of that is irrelevant to the discussion at hand, however. RDM in general needs love, regardless of those abilities. I feel that the best way to implement that Composure/Embolden idea is to keep the personal magic damage buff regardless of which raid buff you use for the party. You'll use the one that benefits your raid the most, which is what I did on SMN when I made that swap.
But that's future talk. Currently, RDM's raw DPS at the moment just needs to be pushed higher. And the only way to go about that is to buff potency values, or alter their mana generation to give them more melee combos than what they have currently.