Hi Hugues,
nice build!!!
I fully support your flight time numbers. My new build with 9.5" props and with 200g payload has a flight time of 36min (auto survey mission) . However, I am wondering about your 3S and 4S comparison, because I was using a 4S LiPo for that flight time.
I am also wondering about the 15A total ship consumption you mention. My build draws about 8A to 10A in average and I thought your build should consume less. But, I calibrated the sensor a while back and I am normally working with larger hexa copter, so I have to double check.
What is the weight of your gimbal and the Gopro together? Also, what is the weight of your 4S3P LiIon battery pack?
Cheers,
Thorsten