I see no inherent dangers in rebuilding your clearos with sub-ohm coils if possible. One thing to consider, however, is that the heat is generally much more than a standard or even low resistance coil on a regulated mod or APV. I don't necessarily believe that they intended clearomizers to be rebuilt, despite it being a possibility with certain heads and setups.
The chance of burning a rubber grommet, insulation piece, or other such component rises when you begin to fiddle and re-make such coils. I certainly wouldn't put a .3Ω coil in a Protank head unless I was absolutely certain the airflow and wicking capacity would mitigate any potential hazard of burning rubber.
That being said, .8Ω to 1.0Ω is a definite safe zone in the sub-ohm realm. At most you'd be pushing 22 watts which, to me, seems well within the intended range.
Maybe... just be careful dry burning them and always check/maintain your batteries!
Vape well, vape often
