Well it is pretty easy to determine when it is worth to use ES with 2 EF, since you simply need to know the damage per LS charge.
But the issue is that those setbonuses really favor big Fulmination, so it might causes issues with EF, depending when you should use it without wasting stacks.
Multi Dot FS
Gain Shitload of Lava Surge procs
Redot FS with 2 EF Stacks
Still get Shitload of lava surge procs while FS deals insane amount and Lvb floods you with LS charges
Yay.