AoE Burst has nothing to do with Lava Burst cooldowns...I'm a touch confused with where you were going there...
LvB is a single target spell, that just so happens to be the one spell with a short enough cooldown and enough damage to outpace a healers self healing capability should we not get interrupted or slowed. Adding to the cooldown of this spell would probably upset balance a bit for shamans should no other class change at the same time. Reducing the cooldown by 1.5 seconds probably won't change PvP much since you wouldn't wear 4 PvE pieces for arena, and BGs are zerg fests anyway. In PvE this should bring shaman caster DPS more in line with other caster damage.

As far as AoE is concerened this will have 0 effect on fire nova, magma totem, or thunderstorm...so I wouldn't push for longer cooldowns on LvB to get more AoE damage.