Am I missing something fundamental here?
SF6 has an awesome global warming potential, true, but it is also very, very dense. My recollection of SF6 is that it notoriously pools in the low parts of structures and in such spaces presents a danger of asphyxiation, ~6kg/m^3 at STP, where nitrogen is around 1kg/m^3 under the same conditions.
I am having some trouble with the notion of a gas that dense managing to get high enough in the atmosphere to matter. At street level, sure, but 10km up at the top of the troposphere? Got my doubts.