don't quite understand you!
does the current exhaust point down or straight?
most diesels point down as they do burn oil (thats their fuel) and a straight one will likely make a sooty mess of the back of your car, and get all over the windscreen of teh car behind. hence why they point downwards.
a free flowing exhaust on a tdiesel engined car is a bit pointless, as yes on petrol models they can allow the turbo to spool quicker and at lower rev's, however you must agknowledge that turbo petrol cars are at a lot higher boost than turbo diesel cars and frankly it'll make sod all difference to performance.