Cannot compare the wattage of a kayfun to a RDA. 1.5 ohm, I'd be shocked if he exceeds 15watts.
With your RDA you could use very large wire that could require allot of watts to heat it up. It all depends on what the coil is made from. A 1 ohm coil from 30g wire would take much less power to heat than one from 26g.
A single coil build, as long as its not a crazy large build, your Istick should have no problems heating it up.