Actually, IMO you can also use this even if you don't have sound set ups especially if your car has a strong modified engine. A car with very good earthing, can have better idle, optimum fuel injection flow, better response, head lights will emit light more efficiently. Basically it gives your car a better electricity flow than oem.
Hp gains are useless, around 1 or 2 only, and they're made not for hp gains actually.

But it is not something that is needed especially on small stock engines, and those w/o sound set ups. That's why you can see these earthing kits most of the time on strong engines. ;)
I just don't know if that 3,500 your saying is really a legit HKS though. I know it is more expensive than that.