The maths on all of this looks fine, until you factor i the drain ability of the batteries. 16340s are likely to be be 2C batteries, which means if they're rated at 800mAH (and what it says on the label is no necessarily true) then 1.6 amps is the best a stacked pair will put out at their stated voltage - anything above that will show a voltage drop. At 4 amps, the voltage could be as much as 30% down on nominal - and the experience is likely to bear that out.
24 watts of heat going into an atomiser - no matter how many coils - would result in blisteringly hot vapour. But, 10C batteries would do the job ( that is, effectively, batteries capable of discharging to lowest safe voltage in six minutes without overheating or showing a fault - properly high drain)
I'm running a 2.6 ohm Dual Coil atty in my GGTS/UFS with the Kick set at 9.5 watts (ish) and it's delivering like a steam loco on full thrutch. That likely means it's a little cool for some tastes, but it's thick, chewy and delicious!!
