They sound good, but they are quiet, and I'm pretty sure it's also a high mount exhaust..
I have the HR1 Riot and it screams. It's only a slip-on. It's stupidly loud tho, and it takes forever to get cause they make em per order. But it's the only shorty can I know of for the 1st gen R6, and it makes the bike look way newer
I'm pretty sure the link in my signature has a video sound clip of the HR1 and my old D&D which everyone hates..
You should be fine if you just get a slip on (no need to rejet). I had a Scorpion on mine and it ran fine without a re jet. If you got spare change then go for it, but it's not MANDATORY.
As far as sound, before I sold it I got a full Leo Vince and that thing sounded nice as hell.