Not quite accurate.
The power capacitor works at any partial throttle setting not just when releasing the throttle. Why would you want to increase motor speed under braking anyway?
You are right about the suppressors though.
A speedo works by rapidly switching motor current on and off. When you push the transmitter stick the amount of time the current is turned off reduces until you reach maximum when the current is on all the time. Ideally the waveform produced by the speedo switching on and off would be square. However in practice the current is gradually ramped on and off.
The voltage a battery can supply reduces with increasing current. What the capacitor does is charges up in the part of the switching cycle when current is low and battery voltage is high. It then discharges in the next part of the cycle when battery voltage is low and current is high. This means that the voltage stays more constant.
Basically the capacitor gives more power at low throttle settings. It also helps the speedo run cooler.
It makes no difference to top speed.