Also, for the record, all I have ever used are the purple eFest batteries. I have six of them, and they all work like a dream. Been using them in the SX and they have worked fine as well. I'm a heavy user, and I am squeezing about 5 to 6 hours out of one battery, around 22,000 joules.
I also received my SX Mini M class yesterday, and I'm loving it! Really fantastic device.
I spent about two weeks, debating in my head wether or not I should pick up the SX. I'm fortunate in that I can easily afford it, but I'm very frugal (ok... cheap) and don't want to spend money on a mod, if it's really not going to be that much better then the istick 50w I've been using for several months.
Well, I can tell you, the SX delivers a MUCH more pleasant vape, especially with the different modes.... I also set up on TC coil, and enjoy that function as well. Love how the ohms are shown to the 100th, love the display of remaining battery life in volts, I could go on and on. Simply a fantastic device!
Haven't yet drained my first battery, an LG HE4 2500 mAh, but my joules are showing (how often do I get to say "My joules are showing", in a sentence

I'll have to see if the joules show a similar number whenever the battery is about to die.