Just checking the math to start with, 5 ohm coil at 7.2v is 10.36 watts, 1.44 amps. 1.5 ohm coil with 3.7v is 9.12 watts at 2.47 amps. So a 1.24 watt difference, I know, I'm splitting hairs here!
A 4 ohm coil at 7.2v is 12.96 watts at 1.8 amps.
Coming from the flashlight world, I'm not a big fan of stacking Li-Ion batteries myself, I've seen flashlights, and mods go boom from battery failure. It can be done somewhat safely, you will need to buy new batteries and label them in pairs, and KEEP them in pairs! This keeps them matched up and they will be charged, discharged and wear the same. But even then, it's no guarantee!
Be safe!