Whenever I've tried the marginal increase in mpg has been more than offset by the not so marginal increase in pence per litre.
However only testing will tell as your driving style will be the main differentiator. Why not run a few tanks and calculate your cost per mile then compare to the standard stuff? its hardly a huge financial risk
I do not think that you will gain any benefits from buying the"premium" diesel. I used to add Millers (little 500mil bottle) to my petrol RS4 when I first had it. It supposedly increased power and mpg. It made me feel good at the pumps with people asking what I was doing but I do not think there was any significant changes to the power or mpg (car mainly sat on the m25 anyway lol)