I'm going to stick with IMR batteries.
They are the safest choice in a stacked configuration. Here is what Cisco posted on another forum regarding this:
It is true that a decent Li-ion (in the larger sizes) will output enough current to run any e-cig properly. IMR's will output more consistent current across the discharge cycle, Li-ion tend to drop off faster then IMR's, the reason the first 10-20% of the discharge cycle on a Li-ion is hotter then the remainder. If your boosting and or regulating you wont notice a drop off in output, so with an IMR you will sacrifice vape time over a Li-ion, for that configuration Li-ion will serve you best.
Personally I prefer IMR's because of the safety factor that Li-ion do not have, I am willing to sacrifice vape time for added safety and consistent unregulated current output. IMR's are a safer chemistry in the fact that if they vent, they only vent with gas not with gas/flames, IMR's do not require protection circuits and are designed to be the safer alternative in stacked battery configurations similar to LifePo4 ( true LifePo4 Chemistry is 3.2v nominal, LifePo4 do not come in 3.7v, it is a chemistry limitation. If you see a LifePo4 advertised as a 3.7v cell, its more then likely LiPo chemistry)