alright new findings, this is with NO buff, NO proc trinket equipped and NO unleash fury:
if you use only lava burst, overload seems to do the correct amount of damage at 75% of initial hit, and dmg does not fluctuate, most of the time 157k then follow by 118k overload, but it does occasionally goes down to 130k somehow.
HOWEVER if you dps like normal, mix in lightning bolt in your rotation, lava burst dmg goes down to 130k MOST OF THE TIME, however the overload still does the correct amount of dmg at roughly 98k
also when you first enter combat, lava burst almost always hits lower than what it should at 130k, then the next lava burst goes back to normal 157k each(if you don't use lightning bolt in between!)
NOW, if you throw unleashed fury in your rotation, the overload dmg starts to get weird, the overload of a lava burst under the effect of unleashed fury is always lower than 75%, roughly 58% usually
ps. i have this addon called Shaman's Friend, i use it to display the total dmg of lava burst+overload, you can use it to help you keep track of lava burst and overload dmg consistency