Woww thats wierd having said that my Bora was literally a custom made 3" downpipe to decat right through to the back box which then reduced to 2.5".
On idle it was pretty much silent and an full chat sounds amazing!
On the rolling road it didnt struggle at all, I had a nice smooth line right through the rev range upto redline.
It might be a good idea to buy a reduced size back box, maybe 2.75" or very minimum 2.5".