You don't have to worry about resistance on a mod like the Sig 150. That's for mech-users to worry about. Dual 18650, 100w+ regulated boxes follow different rules. They usually run the batteries in series and use a buck converter to trade voltage for the amount of current that ohm's law dictates is needed to power a coil with 'x' resistance at 'y' wattage.
Assuming a given wattage setting, this means that whether you run a .1 or a 1.0, the current draw on the batteries will be the same - in spite of the fact that the current going to the coils is vastly different. It doesn't care about the resistance of the coil. It will pull however much current it needs from the batteries to generate that wattage. It's sort of a two-part circuit. In the first part, you only have your batteries and a buck converter. This part of the circuit operates independently of the coils. There's no resistor in the circuit, so ohm's law does not apply. It's only after the power passes through the buck converter that the resistance of your coil becomes a factor. Once the power passes through the converter, your batteries are no longer directly involved in how much voltage and current is being sustained. Current-wise, the buck converter serves as a buffer between your batteries and the atty, which in turn ensures a safe vape on even a .1 ohm coil.
You have input current and output current. The input current is what's taken from the batteries. The output current is what hits the atty. Your output current can surpass the amp limit on your batteries so long as the input current doesn't.
Lets say you drop a .1 on the Sigelei 150w. To hit 150w on fresh batteries (8.4v,) it will need to pull 20A. However, what actually hits the atty is 38A @ 3.9v. That's a safe 38A going to your atty. 20A @ 8.4v yields a little more than 150w. That's what your batteries give. 38A @ 3.87v yields exactly 150w. That's what the converter feeds to your atty.
Another thing worth noting is that as your battery voltage drops, the amount of current that the mod will take from them increases. Let's assume the "low battery" cutoff is 6v. At that point, you will be pulling almost 28A to get 150w, so you will need 30A batteries. The highest you could run 20A batteries all the way down to the cutoff and without surpassing the limit would be 110w.
Essentially, what this all means is that if your resistance is within the range that the mod will fire, then it is safe to use. You should worry more about your wattage setting, though even that isn't as important. You can run most 20A batteries (VTC4's, 25r's, HE4's, and so on) all the way up to 150w and it will only be towards the end of the discharge cycle that your batteries will be significantly taxed. It will hurt your battery lifespan and runtime, but most likely not your face or your hand.
And on tard-ohming... ...I've never understood why people would want to super-sub-ohm with a single 18650. It's not the heat that I worry about, nor would I fear an internal short in the battery, as the internal resistance on the batteries we use is on the order of milliohms. Its just that the sag on a .08 draw has to be insane. It's probably kicking out far less than 3.7 volts under load. I can't help but think that the performance can't actually be better than something more within the limits of what the battery can handle.
Super-subbing on a tube mod just seems risky, impractical, and ineffectual. The power boost can't be very high compared to the strain placed on the battery. There has to be a point of diminishing returns with regards to current draw. Past a certain point, the increase in current will stop making up for the drop in voltage, which would mean that not only would wattage no longer increase, but actually decrease as the resistance gets lower and lower. I'm not sure how low you would have to build before this would happen, but I suspect that a .2 with 9A of headroom would probably outperform a .0x that even a true 30A battery would be struggling to power.