There are 3 issues with sub-ohming a "stock" Kayfun.
1. The insulator - The ones on most clone Kayfuns cannot handle the heat associated with sub-ohm builds.
2. Air flow - You need alot of air flow with sub-ohm builds to get the kind of vapor that a RDA produces.
3. Wicking - to run sub-ohm builds you use alot of juice, with out enlarging the juice channels you are not going to get enough juice.
That being said I have built a Kayfun as low as 0.3 ohm. I had the juice channels dremeled out almost double the "stock" size but it still didnt keep up with wicking. The build was only good for short 3-5 sec pulls because of the heat.
Hope this helps