I dont have hks, but i have full turbo back 3in apexi exhaust with a tesudo muffler. Nice deep sound.

Here's a bunch of reading on that.
http://www.dsmtuners.com/search/34973/?q=hks+exhaust&o=relevance

Which kit are you planning on getting? Are you keeping the cat?
I'm sure if you stick with a quality brand name you will do alright. Maybe check out what some of the supporting vendors have available.
Some have had luck with even the cheap ebay exhaust that costs alot less. Then others have had to modify and/or weld them just to get them to fit.

I like the thermal R&D and have had it on the last two cars. First one was 2.5" dp to 3" thermal no cat. On this car full 3" dp and thermal no cat. I love how it sounds.

I'm currently using a thermal R&D with a 3 inch downpipe w/ high flow cat. It sounds great! Not too loud and creates a throatier sound compared to most exhausts that just give it a deep rumble.

HKS hi-power has a deep mellow sound, and isn't too aggressive. However it isn't the best flowing muffler, but good quality.

I have a 2.5in Buschur DP to RRE 'stealth' cat(it's hollow) to a 3in Thermal R&D catback.
http://tinypic.com/r/357lzqx/8 <--click for sound clip of this setup. Site will not allow me to embed the video.

I run a Borla Downpipe & a SR-S Catback & I love the sound & it still look's great to I have had it on my GS-T for about 4 years now.

I had a hks on my first gsx. It had a nice note and fit and looked good.

3" turbo back with no muffler in aluminum. Light and nasty! Plus, kit exhausts are for suckas.

I have full apexi n1 it starts as 2.5' and it gets to about 3.5 at the end looks and agressive and the sound is deep.

Pipe is pipe for exhaust, ebay will be just fine. Some of these prices are outrageous for a 3" pipe. I think I bought a megan exhaust and it sounds just like everyone else has described.