My Evo 6 has a HKS SuperDrager, DeCat pipe, Blitz induction kit (until recently had the HKS one, which was quieter) and also the HKS SQV dump valve.
Noise limits vary depending on db level and at what rev speed.
Goodwood was 96db at 5000 rpm, which registered 90db. At Castle Coombe I registered 86db at 4500rpm!! I did get pulled over at Combe because they said I was averaging 96db (higher than anyone else) but it was induction noise. I doubt it was me, but I was playing with an STI scooby which made similar noises, so between us, we were noisy!
Your car may be noiser depending on how new the HKS exhaust is. They have fibre matting inside which can break up and get noisier over time. A 5 at Goodwood was noiser than mine, but well below the 96db limit.
Hope this helps.