Unless you are running something like 1 bard 3 magical dps it wouldn't be worth it to lower your dps for increased song duration. For the Second Coil of Bahamut turns 6\7 have dps limitations between the phases and lots of movement and positioning you would not gain a large benefit from it outside of a few quick phases that your current mana pool could support. The only turn I would see it being useful would be turn 8 if you ONLY ran one bard you might have a solid reasoning, once again if you are caster heavy.