I buy fuel additives more for their cleaning properties, lubricity and safeguarding against potential water contamination in the fuel than I really worry about an immediate boost in performance or cetane.
I will say, I have owned a number of TDIs that were previously neglected have their performance improve noticeably over the course of a couple tanks of fuel with a good additive like one of Stanadyne's, Howe's or even Hot Shot's, and also driving the thing hard. The combination of extended drives under high loads with a good cleaning additive does absolutely restore performance that has slowly been lost over time thanks to coking up of the tiny holes in injectors.
The cleaning properties combined with the improvement in lubricity will not only restore performance with regular use but also help your fuel system (injection pump or high pressure fuel pump and injectors in particular) simply last longer while also performing better.
I can't tell you how many cars I've personally owned that had slightly rough, smokey starts and just overall didn't run as good as it should, and have it turned around by using a good additive and driving the car hard as often as I could.