Even 2 seconds of downtime is more than any other ranged job would need to have in this scenario.

Consider that in 2.0, BLM damage came from instant cast procs, so you could proc and then use Aetherial Manipulation basically whenever you got one at ZERO damage loss. Now because of the hardcast nature of Fire IV and the significantly lower number of total procs, weaving off-GCDs is a huge pain. This is also half the reason why mana shift is worst used on BLM.