[BrianG]

A higher voltage cap won't hurt anything at all. The higher the voltage and/or capacitance, the physically larger the cap is. While you want some voltage overhead (at least 25%), there is no benefit by going higher. A cap will only charge up to the voltage being applied to it. Applying 12v to a 2,000v cap will only have 12v on the cap. A 10v rating is more than sufficient for a ~6v BEC. Low-ESR type is not really needed IMO (but won't hurt) because there aren't a lot of high-current ripple. Capacitance value from 1000µF to 2200µF should be enough - if you need anything higher, it's likely that the BEC is simply working too hard.

Another option is wrapping the throttle cable through a ferrite donut. This would only help if high-frequency signals are being induced into the BEC wire (most likely from close motor wires) causing erratic receiver operation. This won't help with BEC voltage sag - although a receiver "brownout" could actually be the receiver "rebooting" from noisy power.