I'm not too sure about running it without a gearbox but I am using the e-flite 400 brushless with the 6.6 - 1 gearbox with my ultimate, it flies great plenty of power with a 10x4.3 prop using a 3 cell kokam 1500mah battery pack, the 2 cell just doesn't give enough power.

I too am using the electron 6 receiver with hs-55 servo's and the CC 25 ESC.