I've got a HKS Hyper but with a single 5" cannon tip. Mine has an adaptor that you can put in the cannon tip to 'choke' it. I've never used it, but it would be good to go if I was ever done for noise.
HKS Hyper exhausts I thought were 90mm outside dia. so yeah, 80-85mm ID. Cos it's a straight through pipe from the cat, and given the diameter etc I'd have thought it was a pretty good performance pipe, I've never had any issues yet but I was only running with 240, now I'm aiming for sub 300 and I guess I'll be testing the bounds [3076, hks cast low mount, hks std wg, custom dump front plump back and custom s/s intake eventually].
BTW the exhaust has a good deep note, but is pretty quite really, I don't get headaches on long trips .