TGY-306G-HV High Voltage Ultra Fast/High Torque MG Digital Alloy Cased Servo 3.7kg / .05sec / 21g This servo looks a awful lot like the MKS DS-95 for a third the money. Source: http://www.hobbyking.com/hobbyking/store/__24764__TGY_306G_HV_High_Voltage_Ultra_Fast_High_Torque_MG_Digital_Alloy_Cased_Servo_3_7kg_05sec_21g.html
It looks like it would be a great tail servo for your helicopter because it seems really fast. It costs almost the same as the stock E-Flite servo so how can you go wrong?