it's pretty significant... I personally liked the sound with the stock as well. I dont think you can go wrong with the RB. It will be quiet for regular driving but when you get on it, it really picks up. If you get it, like it, but want a bit more sound the supercat isn't all to expensive and also helps the sound a bit. I don't have a video of the car before the supercat so I dont have anything to compare to really. Here's the video just revving in place...
http://www.youtube.com/watch?v=C5wSfH8Q9p4
It's a little bit deeper in person... but video isn't all too inaccurate.