I don't know about on the 3.6, but on the 3.2 the Borla sounds great. Not loud at all, no interior drone (you can barely hear it inside), but it has a nice growl when you get into it. Personally, I think the tips are about 1" too long, and too large in diameter, but that's me. I like more of a stealth look, like the CTS-V tips.
I know it's not the 3.6, but I have a video with sound of my 3.2 on the dyno with the Borla. I'm not sure if it helps or not. (the label says it's a 3.6, but I can assure you, it's a 3.2L car). My car it some kind of limiter around 120mph, so it wouldn't rev any faster, which is why it goes for a while without accelerating.
http://cadillacfaq.com/faq/video/odydyno.html