24g Kanthal 2mm dual coil for 0.2ohms gives me these figures and at 15w would be a very cool vape as you can see from the Heat flux:
So I was very dubious to say the least the only thing I have which is similar build is my Doge RDA dual 0.26ohms so I tried that on my d2 and I get a warm vape, so upped it to 20w exactly the same, 25w the same, so methinks that the mods are probably not allowing you to vape at 15w but rather the minimum wattage allowed for the build even though it is showing 15w it is not stepping down as it should, suspect the ipv2 is the same

Not even sure how flexible things will be if they do still sell ready made after the cut-off - may be in my interests to get my nicotine use as low as possible and just make my own.