If you are sticking with stock servos I would leave it be don't really need it. I only used one with my hitec for the 7.4v
A castle cap stores energy and helps with voltage ripples and surges
I used one on my mmp with a 9000kv motor on my merv but you wont need one with the stock setup
I agree with Dave (sorry, 50togo), mine caught fire after 100+ 6s runs. This esc has about 50 5s now and is fine, it was a fluke. Don't bother with either, but a cap pack can make less work for the esc with ripple currents.
I reject your reality and substitute my own!
Seems we are going in two differnt directions here.
Caps essentially serve the same funtion on the input and at the onboard BEC output, smoothing voltage.
I see it referred to as ripple but thats not technically correct. Ripple on a DC power system is the introduction of AC voltage onto the same, another discussion altogether.
At the input sags are introduced by two primary components. Batteries that are not up to the job or wiring and connections that cant deliver. A cap bank is a work around to store energy and smooth out the DC input as the ESC switches. If you have batteries that can deliver and the largest/ shortest wire to reduce the impedence at the input it will reduce sags. When you get into ludicrous power delivery the entire system by design is insufficent. Take a look at what even #10 wire is rated for on current delivery, a whopping 40amps when you are talking about the type used in RC cars that is multi strand similar to type DLO. Granted that the rating is assigned for 100ft runs but still thats insuffcient. Running higher voltage packs reduces the sags at the input as ohms law dictates when voltage goes up, current goes down for a given load. My thoughts are running 6s is at the limits of what the transistors in the ESC can handle. Being an engineer I never run anything to its max rated limits as its going to fail sooner rather than later running it no more than 80% of its max rating. 5s in a system with a max rating of 6s is just a tad over the 80%. If I want to run 6s Im going for a system with an 8s max rating.
Just my 2 cents, take what you like from it and leave the rest.