regardless of the setup you use IMHO you do not need a powerbox system on the Flash. it's a pretty simple setup and the only reason i recommend the sensor switch is it's dual batteries/switches for redundancy.

on the sensor switch, if used and the maximum current is exceeded it just bypasses the switch (you get full unregulated power). if you're drawing 15 amps by the servos they won't be harmed by the 7.4V direct connection and as soon as the power consumption drops below the regulators ability it'll kick back in and the servos see 6v again so worrying about the current capacity of the sensor switch shouldn't be a concern to anyone, and I'm not saying it's the best option for everyone, but they work great in these simple setups where you want redundant switches/batts

on the new JR setups, you dont need either.