If you have steel rear dropouts you might be ok, but as the power goes up the more likely you will spin out your dropouts from the torque. If they are aluminum you really need a torque arm. I would definitely get/make one at the power level you are running at, but you might be ok. Over time you might spin your hub.
http://endless-sphere.com/forums/vie...lit=torque+arm
http://endless-sphere.com/forums/vie...lit=torque+arm