I was talking in terms of current output, the OKR-T/6 has 6 Amps versus 3A for the other parts.
You're not going to get 13 Volts out of a buck regulator unless you apply 13V plus the dropout. So, with two 3.7V cells, you won't be able to output any more than 7.4V less droput. I don't know about the part you linked to, but dropout for the DimensionEngineering part, is 1.2 Volts. When that one is powerd by two Li-Ion cells, it won't output a whole lot more than 6V. I don't know exactly. Someone who has used that part could tell you for sure.
I suppose you could power your part with 3 cells if you wanted to output higher voltages, but running an atomizer over 6V doesn't make a whole lot of sense to me. If the 12W you get with a 3Ω atomizer still isn't enough, you could use a 2Ω atomizer and get 18W. Though, I don't see how vaping with that much power would be anything but unpleasant. Even the dual coils (which handle high power better than anything else) put out really hot vapors at 6V.