Did you also try different timing on your ESCs can make a difference ! I also have a CX WKM but i have a herkules board (FW off) inside and i get around 12 13 mins indoor hovering with a 4s 6000 mah, axi 2217/20 11 x5 graupners, copter weight 1.8 kg, probably could get more, but i dont squeeze...