Lol the efficiency police is in town!
Well if you get bored of flying after 15 minutes then the Mini talon QP isn't for you!!
We've had ours go 137 minutes and 115km on two 6600mAh 4S in one flight.
You could takeoff, send it on an auto mission, go home and have lunch, then a hour nap, then afternoon tea and still make it back in time to watch it land!
As for the prop setup the added weight on the talon increases the cruise speed (dictated by the wing loading and overall drag), so you need to have a prop pitch around +130kmh. Our 10x8 Aeronaut prop is effectively overproping it to get ideal cruise...this is by experimentation not just calculation, using that same Cobra motor. You only need around 450g of thrust at 24m/s, so provided the airframe is nearly as slippery, I'd try to stay around those sizes.
I recommend to buy a prop pitch up or down to test. The test is simple but a bit time consuming; which prop maintains a given cruise at the minimum current draw. Set it to 4-5 different speeds to test best cruise (even out of the range you'd think will work) and do that same test for all the prop pitches you have available to test. Then extrapolate the best range relative to velocity and current draw to choose the right prop. Set your cruise speed accordingly and fly longer!
Sorry, I get an itch if something's not efficient. The German in me I suppose.