Very late for a reply here people but it think that it depends what you are looking for is it the improvement in power or just the exhaust note.
Free Flow exhaust ( FFE ) System is said to increase the performance of the car by about 4-5 percent and to give a good sound of exhaust but all this is applicable to the Petrol Vehicles. But when it is diesel it is completely different.
My family has previously kept petrol vehicles and have some times got the FFE done for it. But recently due to change to Diesel i do miss the sound and performance of the FFE. So i went to my Exhaust specialist and asked him if it was possible to do some thing for the Diesels. He told me that it is not possible for a diesel vehicle due to the presence of a Turbo.
But however he said that there is an installation in which he removes the Cat Converter of the vehicle and replaces with a Component which does improve a bit of throttle response and does reduce the vibration of the vehicle up to some extent.
And if a person wants an exhaust note he can always change the end can of his exhaust.
Here is the Link of one of the members of TAI
who has changed the end can of his VW Polo TDi. The Exhaust note wont be as good as a petrol. https://www.facebook.com/photo.php?v=543518322400032