Mike
Think about it, Although this scenario is not exact and my technical translation is not all that great, some of it will make sense. if you just proportionally increased the size of your copter from a 4 to a 6 motor copter, you are adding a bigger frame, two more arms, two more motors, etc.... (weight!!!) Now if you weight grows fairy proportionately with what you are adding.
Just adding two more motors = roughly 1/3 more power use, which if you where getting 15 min of 4 motors, now you have introduced another 1/3 of power consumption into the mix. 1/3 of 15 min will = 10 flight, thats if everything was consistent, but there are other factors that will play a role.
Another consideration is to take in is the thought of how efficient are the motors to prop relation. Now you are getting into efficiency, each motor will have its own sweet spot, where it will give you its max lift while using the minimal amount of power needed to produce that lift, however as you go above the point the amount of energy used will increase, sometimes exponentially, and in that case you will get a huge waste of power per unit to the amount of lift being produced.
In you case to me it sound like whatever is going on you are above that most efficient spot.
Lately the way people are adding flight time is using new motors, motors with low KV (KV being the power unit needed to make one rotation), DJI Motors are 920kv, I am not great of explaining the relationship but in general the lower the KV larger the prop the motor can support ???, the larger the prop the more thrust. So I have seen someone take a f450 extend the arms, add a low KV motor, put 14 inch props on it, lift a much heavier load, add two 5s (5000mha) batteries and fly the copter for 24min, of safe power time.
At the end of the day, Its kinda like the old Sims City, A lot of combinations can be done, you just have to find them and understand the limitations involved in creating them.
