Old thread, but right up my alley. I've seen coil life go down on Kanger single and dual coil heads and Aspire Nautilus dual coil heads. The problem I'm detecting is that they vape great for a couple of hours and then the coil resistance begins to rise. A 2 ohm head slowly becomes a 2.5 ohm head, then a 3 ohm head. If you continue to raise the voltage to match the resistance and keep the vapor level up the coil will fail open when it approaches the 4 ohm point.
I've taken a number of these failed heads apart only to find that one of the non-resistive legs has broken off. The solder joint will still be attached to the end of the leg, but the coil heat has fatigued the solder so much that the joint develops resistance higher and higher until the leg disconnects from the end of the coil.
I rebuild these using 32ga Kanthal with a cotton wick and they work fine, just like everyone else in the thread has stated. I might see a half an ohm rise over days, but that's about it and I can clean and lightly dry burn to restore flavor and vapor. I agree that non-resistive wire keeps the heat off the post insulator grommet, but those come in bags of 10 and 50 these days and we can replace the grommet when we rebuild the head, so that's not really an issue for me.
I'm just wondering why Kanger and Aspire don't use a wire weld on those instead of a silver solder joint. I guess it's faster to solder than attach the legs with a capacitive wire welder. I've seen one of those that fits in a plastic project box. Vapers are even making them out of cheap flash cameras. It's the way to a lasting NR-R-NR coil wire.
Here's one of the videos using a wire welder made from a cheap camera. It's pretty ingenious for doing the job cheaply. It you use NR-R-NR coils to save your grommets, this looks like the trick. I may just build one myself. If you click "Watch on YouTube" there are many more like this in the sidebar.