Before getting to the servos have a look at your servo saver. The 3903/5/8 Emaxx servo saver is commonly pretty loose hurting performance. Make sure you have the stronger black spring (chrome is weaker) installed. If you have the black installed and the saver is flexing when just turnig the wheels on the ground consider doing the dual servo saver mod. When I had a 3908 with a single Hitec (333oz) it would flex the saver so much it wasn't funny. Unfortunately the dual saver mod is the only way I know of to stiffen up the saver, which they (or the aftermarket) would've made it adjustable.
Your current servos have decent torque but they are slow. I personally recommend under .18/sec, more like .15/sec would be ideal. According to the servo's ratings you should have enough torque though. I still run the same Hitec 333oz servo in my heavier LST with Trenchers and it throws the wheels around like nothing. The flexy servo saver was the limitation in my EMaxx