If you keep your ohms at .5 you should be ok battery wise if its a true 20 amp...I cant confirm the 2100 efest is in fact a sony vtc4 re wrap so i wont say its ok...If it is a vtc4 and its authentic then it is a true 30 amps..Again i cant confirm that so the answer is no for now on the efest
The other issue with a mech tube mod is the firing switches...Regular routine maintenance and cleaning are necessary but this still doesn't eliminate possible shorting if you dont catch a problem before it happens..which is the dangerous part...The same goes with your freakshow rda or any rda, tank, cheap clearomizer etc.....A simple mistake can be a big problem with no safety features of a mech tube mod.
If you MUST HAVE a mechanical mod then check out the Dovpo GX200 box...This has alot of safety features built in to make it a much safer device to use...You need 2 batteries however but they are wired in parallel which will give you double the battery life you get now with a single 18650.
You can still use your .3 ohm coils with this if im not mistaken.
They run about $35 dollars and up unless you wanna wait a month to get one from china which you can get for less than 30 dollars.
Its really a nice box for the money and this is something im looking at myself as i dont care to risk using a normal tube mod.
Pick up some Sony VTC4 batteries to use with it and dont use them in anything else but the box mod...youll have 4200 mah of battery life.
Im personally gonna use 2 Samsung 25R's (True 20 amp) as i dont plan to go below .4 ohms and want that extra piece of mind...5000mah