Engine sounds fine for the Mayhem. Not sure about the 3050 servos. I used the HiTech 5645 on the rudder and elevator and the 645 on the ailerons. They are plenty powerful. I think you want at least 90 ozs torque on the tail servos and probably about 70 or more on the ailerons. Anything less than that may not be enough. Battery sounds fine and with the 6 volt you'll get more torque out of the servos (I'm assuming they'll handle 6 volt. Some Futaba servos don't)
Anyways, hope you enjoy this plane. I sure like mine.
Thanks
Barry