This isn't related to exhaust, but someone else mentioned Ziebart on another thread today so I'll give you my 2cents too!
You can try the Ziebart treatment that not only is supposed to help prevent rust, but it is also supposed to add an additional sound barrier on your underbody.
Try
www.ziebart.com to see some details. It cost me around 300 or so for both rust & underbody and I'm satisfied with the job.
I don't think you want the corsa touring either if you are concerned about sound...check their (corsa's h2 page) sound page for some samples. I have the sport and love it, but I wanted a loud exhaust.