My 2c in this hot subject
. Trust me I'm electrician
or was it politician
Capacitors store and release little energy VERY fast.
If connected across the batteries the effect is that the voltage into the ESC is slightly higher under full throttle from dead stop, that can prevent glitching. I have no idea how it could improve midrange, but maybe it does by allowing some ESCs to work slightly differently.
If connected into the receiver it can prevent glitching and make your personal transponder more reliable, especially with some combinations of ESC and digital servos.
If you never get glitches and your PT counts perfectly you don't "need" capacitor.
Capacitors store very little energy so you'll need a big one.
Capacitors charge up to the voltage applied to them, no more. They charge fully in less than millisecond (1/1000 of a second).
There are hundreds of capacitor types with different parameters. What you'll "need" is the liquid type in a small bucket
, (I don't remember the english name) that has high capacitance, usually >4700uF, low voltage <16V (6.4V is fine for receiver capacitor) and low ESR (that's the tricky part. Most personell at Radio Shack have no idea what this means. Lower ESR means it works faster.
). You don't need to buy the capacitors that your ESC manufacturer sells. Electronic equipment shops should have higher quality japanese capacitors at lower prices.