My dad and I both have Corsa's. For just driving around town, they are both quiet as stock. They do idle a bit louder and are a shade louder on the outside, but inside they sound just as quiet (if not quieter) than stock.
When we've done a side by side WOT acceleration run, his Extremes were louder than mine and have a more aggressive/exotic tone.
Here is a vid of them side by side, not the best quality but still gives you and idea of the difference between the two... you'll hear mine first (red GT/CS).
http://www.youtube.com/watch?v=eJXnwJF2zjA
Since I've added the supercharger, mine is now louder than his which I assume is a by product of the greater airflow due to the s/c.
At WOT, from my experience the Corsa's are as loud as almost any other chambered system out there. But none of the Corsa's drone at all... they are great for those of us who want a more civilized exhaust tone.
Personally, I took a leap of faith when I bought my Corsa's in early 2008 as I'd never heard them in person. I have been absolutely thrilled with them from day one and will never have anything but Corsa's on my cars from now on
