Also mah has nothing to do with amp draw capability.
Technically not true.
Amp Hours x C rating = Maximum Discharge Rate
I just got my first Reo Grand. If you check on the Reo forum, I believe you need to stay at .8ohms or above until they get the sub ohm upgrade. There are lots of sub ohmers on that forum and they can advise you exactly on the Reo sub ohming.
Your fine until you go below .5Ω on a Reo. Below that will begin to sag or full out collapse the fuse spring in the Reo. (safety built into the Reo)
If you go below .7Ω you just need to maintain the firing pin more often with noalox to prevent arcing and voltage drop.
I
vape .6Ω
coils on a Reo everyday with no problems what-so-ever.
A forthcoming kit with an improved copper firing pin and more efficient fuse will solve all of these minor issues with sub-ohming on a Reo. You will be able to easily replace the firing pin and spring in existing Reos, and all future Reos will ship with the new parts. Rob (the maker) is just waiting for the parts to come in, so any day now...
On any issues regarding your Efests; You'll be fine and safe with them above .6Ω However, the voltage drop on the Efests at higher amp draws (above 4 amps) is
horrendous from what I've seen. In other words: they hit and perform really weak on a mech. You'll be switching out those batteries because of poor output performance way too often regardless of their higher 2000-2250mAh. I'm not sure about the newest 30A Efests (purple) though, they haven't really been tested/charted yet due to being so new...