Capacitor is meant for devices that are hunger for current, specifically subwoofer + amplifier. In theory, Capacitor will stabilise current and remove electronics noise, hence improve all electronics components performance in your cars. Capacitor will compensate a sudden high current lost, especially on heavy and loud bass. It will also resolve the light dimming issue when you are on loud music, meaning regain original stable brightness. 

As for Voltage stabiliser, it is actually an Impedance stabiliser. Meaning it ONLY stabilise your load to your battery. So your lights will dimmed but stable irregardless on loud or soft music. In fact, Voltage stabilizer is best for engine ignition and protection to your battery because it stabilise your load. 

Grounding cables are to reduce noise to electronics components. VERY important for ICE, especially those using CHINA amplifiers. Tweeters are the most sensitive speakers among all, hence sound quality will improve too. These noises are produced by the alternator during charging. 

As for all the above will increase torque, more horsepower, blar blar blar are all indirectly agreeable but not a significant improvement. 

Capacitor --> Stabilise current and compensate current lose 
**Current is known as Ampere (A) 

Voltage Stabiliser = Impedance Stabiliser --> Stabilise load 
**Impedance is known as Ohm 

Grounding --> Eliminate noise 
** Noise is know as Decibel (db) 

Conclusion is all the above 3 devices are meant for 3 different things. 

Pls correct me if I’m wrong. Thx.