I've been checking the exhaust on my car and it looks like it has a 3" system which looks like its been de-cated up to the point after where the cat would be then it goes to a 2.5" prodrive system for the rest of the system but my question is if I changed the rest to 3" so it was a full 3" system right through would the cars map need tweeking?

 

I think the only issue you might come across would be overboost due to a 3" system being a little less restrictive. Best be on the safe side and get it re-mapped, you may find you gain some more ponies.