Stefan,
I replaced the entire center section of my exhaust system with 2 1/2 inch, from colectors next to gearbox where system went into first long silencer through to rear silencer (will replace standard rear section when it rots out), but just removing all of the center section and replacing with straight through pipe had no adverse affects and had the bonus of very nice sound! (a little like the old GRP A 635 race cars if you ever heard them) and not to loud
steve