I wanted to chime in on this thread, because there is lot's of good advice here. As mentioned by others, sub-ohming can be dangerous if time is not taken to calculate what you are really doing. If you push a battery beyond it's working limits, it will explode. That is a proven scientific fact. But, the science behind it is not all that complicated. The math is actually quite simple, but there are apps out there that take all the science right out of it making it even easier. I've been using Vaper's Toolbox as well. It is straight forward to use and all the tools you need are right there in one app. You just enter the gauge of wire, # of coils, ohms desired, diameter of coil/s, coil lead length, and volts, and it tells you exactly how many wraps you need and how many watts you will be vaping at. Just know your batteries and don't build below what your battery can handle.
With this being said, I have a K100 with a purple Efest 18650 and a Tug Boat clone. My build now is triple 28g twist wire, dual 7/64, 4 wrap coils testing at .25 ohms. This particular build is only drawing 16.8 amps and a respectful 70.5 watts, which is well within the Efest's capabilities.
Just use an app and be smart and you'll be fine. Just 1 word of advice on the Efest batteries, make sure it's an authentic Efest by looking for the holographic seal.