you may want to check out the Mugen exhaust it is not loud has a nice note and has ok flow 63mm. My turbo is in the process of being installed and I'm going to run my mugen.

 

do you have the ability to read a/f? i personally would go stock exhaust. You will lose a little peak horsepower, but you may be surprised on how little you actually lose compared to what you have now.

 

I went back to stock and it ran a little rich but nothing you coulndt live with for a short while
what you want is an active baffle to reduce noise below 6k and go full on when needed, as the exhaust is not handling full gas flow at lower revs its probably not going to reduce the flow enough to cause fuel issues.

http://autospeed.com/cms/title_Butterfly-E...02/article.html
bit crude but you get the idea