I had the HKS Hiper system first and I was told that that would be too loud but it was to my taste. Sounded best under over passes and in tunnerls.
However, I switched out for the more stock look. I went with resonators and high flow cats. I dont think it is loud and even a friend of mine who used to race an AE86 said it is quieter then the HKS. It fits its looks in terms of sound. More mature but still a bit beefy.