Wow! That's Crazy!
I Was curious since I normally fly with two 3S 5000mah Hyperions, so I went out today and flew several batteries, one at a time. Temperature was -12C, & I'm flying a 550 with stock motors & ESC's and 10in. factory props. I have a Zenmuse and GoPro mounted, a Fatshark 700 line FPV camera, Fatshark Vtx and PhotshipOne skids. I didn't do the same thing as I normally would re: keeping the batteries warm; I just tossed them in my flight bag before I left the house
Of the 5 batteries I flew, the shortest flight time I got with both cameras powered was 4 min. 56 sec. and the longest was 6 min. 17 sec. I'd expect somewhere around 6 1/2 - 7 minutes of flying time on one of these batteries in summer temps.
You may want to try keeping them as warm as possible prior to flying, and try to insulate them if you can in flight. As I said in my earlier post, I made a thin neoprene sleeve to stuff them in before I fly them, and it works pretty well to keep them warm for the few minutes they're in the air. They seem to generate enough heat from discharge to keep them nice and warm in flight.