FWIW, on my iStick 20W with the Nautilus Mini 1.8Ω coil I usually have mine at 12-13W and I get about 8 or 9 tankfuls before I start noticing the drop in flavor and the metallic taste of the coil.
My juice is 80/20 PG/VG. I just did a 6 day Tiramisu marathon on that same setup I mentioned above to see how long my coil and taste buds would put up with it and got those results above.
Have you tried the higher wattage yet?