On the MVP and VV/VW the circuit protection kicks in @ 1.2ohms, I've heard people say it will fire 1.0
coils, but I have never confirmed this personally, plus coil rating is not always spot on +/- .1-.2 is pretty common.
SVD, VTR and 134 will fire down to 0.8ohm coils.
Not sure what the coolfire 1&2 are rated to, since they are my least favorite Innokin mods so I haven't played with them much.