i have the hks silent exhaust with decat and it not overly loud its just perfect to be honest, nice and noticable but not ouch my ears hurt loud.
and n/a cars are generally louder because they have nothing to block the noise if you remove the silencers, turbo cars obviously have the turbo to block sound so thats why there generally a bit quieter. but in my experience my decat didnt make it hugely louder, see what the others say tho with different systems really depends on what you got
5zigen pro racer and zep racing de cat sound sexual!!