When dealing with servos making 470-630oz-in of torque, the BEC built into the ESC struggles. In part it's because the BECs are rated optimistically by manufacturers (e.g. under ideal conditions of cooling seldom, if ever, seen in the real world). So when it's hotter than in the lab, especially without the 5mph cooling breeze with which they're rated (read the specs, they tell you), the current output goes down. Way down. The other thing is, the connector on the lead is rated at 5A - so regardless of how much current the BEC can output - the real world limit is that connector going into the receiver. 5A and that's it! Fortunately, there are ways around this problem.
We've seen all kinds of attempts to resolve this. Some involve monkeying around with powering the servo directly, e.g. bypassing the receiver. But this is a needless complication. Another way involves forking over for a pricey high power BEC. But in our opinion, the ideal solution involves just using an inexpensive LiPo.
Once you commit to powering the electronics with a LiPo instead of a BEC you're basically treating treating the system like one used in a nitro powered model. This also totally isolates propulsion-electronics from control-electronics. The benefit of this is your delicate receiver and servos won't ever again see voltage spikes or back-EMF coming from the motor, which is better because they're not designed for that. Note, all ProModeler packs from 1000mAh and up come with two leads to overcome the 5A limitation. Other manufacturers offer packs built this way, also.
As for hooking the two lead pack up? It's easy! Just remove the + wire from the throttle lead and that disables the power coming from the BEC to the receiver. Next, connect the battery to the receiver and you're in business.
How do you actually connect two leads to the receiver? That's easy too. Remember, the point of two leads is this allows the receiver to draw up to 10A (recall the limit is the plug itself, which is rated at 5A). By using two leads (e.g. two plus) the whole thing can draw 10A (5A+5A). Little known fact, packs with two leads were our very first product at ProModeler although at the time, the reason had nothing to do with exceeding 5A.
Anyway, to make it easy to power up the system, just add two short extensions to the receiver. Plug one into the BAT-port. Plug the other at any free channel on the receiver. And if you don't have a spare channel, use a Y-connector (one side for the servo and the other for the second lead then connect it to the receiver). Oh, and leave these permanently connected to the receiver.
Now when you connect the battery, you do it conveniently at the extensions instead of poking around at the receiver. Ones from 3-6" long work fine.