Just install a BEC so you can get enough juice to your servo.
Castle Neu 1520 on 6S LiPo Powered Gmaxx (Nitro Killer)
Predator with OS .21TM
Supermaxx with Mach .26
Revo with OS .18TZ
Kyosho ST-RR Conversion
Ofna CR with Tekin ESC/Motor (2)
Well, for $30, I can get a servo that is a step up from the stock one anyway. I'm just wondering why have the built-in BEC output set at 5V instead of closer to 6V? I can't think of any good reason why less than 6V would be preferred.