Fire elemental always resets during first 2 min (Hence 100% uptime) and less gcds wasted on replacing fire totem, since fire elemental has a 2 min duration and searing only 1.
Some people say its useless, but it does increase dps a little because when you hard cast lava burst it becomes "on cooldown" after you cast the spell. Where if you instant cast it, it goes "on cooldown" straight away, hence making the cooldown 1.5 sec shorter.
Also, that instant lvb buff is called "volcano" and seems like i procs even if lvb is not on cooldown.
Overall, 4-piece bonus doesnt seem to be very good, but it definitely isn't useless. And 2-piece bonus looks like a nice dps increase especially in AOE fights (at least if your elemental doesnt go ape!*@! on something that is not even part of the encounter)