Best combo that I've worked out for the most flight time is 1038 dji props on 3s (std dji motors).
The lipos I use are A spec nano's 4500mah from hobbyking, these lipo's are the secret for 14 mins flight time. My f550 weighs 2.4kg (inc 2x above cells in parallel).
Tempted to buy the same motors you have to see if it would give me a little more power with the 3s.
I used to run 4s 5000mah with 9x5 graupners, this would give me 8-9 mins of flight. Plenty of power though, night and day compared to 3s power but now I have the tbs discovery I don't need power with the 550, just nice smooth Ariel footage stuff.
You are right to be scared to fly it with no fail safe! I wouldn't, I have a 9xr on the way to me, should be with me this week. If you want you can have that off me for what I paid from hobbyking.
Purchased it just for a spare.
I also have a fr sky djt module and plus 2 rx spare.
