I don't think you have a large enough prop or enough volts for this motor to make it's optimum power. Hobby Lobby's website lists a 13x8 when using a 4-cell lipo. When using fewer cells, motors actually need even larger props for best power. They list a 14x7 prop for 12 nimh cells. An 11x6 may be a good prop for 5 lipo cells in series. Give this motor the correct voltage and prop and it should be at least equal to that glow engine.

http://www.hobby-lobby.com/brushless-axi2826.htm