14.3v i will keep testing
The low temperature would, of course, had an impact on your flight time but having said that 15 minutes in the air without the weight of the camera landing at 14.3v isn't too good
on the face of it. Forgetting the -8c temp for the moment, I would have expected to see 20 minutes in the air without a camera. One can only assume, then, that the low temperature was significant in your flight time experience. Overall, then,
factoring in the low temperature, 15 minutes in the air and on the ground at 14.3v without the camera weight would seem within reasonable performance from a battery operating normally. Best thing to do is have further test flights without the camera in warmer temperatures. If you can get 15 minutes in the air without a camera landing at 14.5v in temp 15c then your battery is good.
The best flight time I've ever had was 18 minutes in the air landing at 14.3v with camera mounted in temp around 20c
All the above is subjective. Much depends on your flying skills, whether you are aggressive, wind strength, altitude above sea level, etc. For some people, just 10 minutes in the air could also be reasonable to expect from a normal battery.
Footnote. Landing at 14.3v should be avoided. A bit low voltage for my liking...not good for a battery's longevity.