The only thing that's worth adding to pump diesel is something that increases the Cetane rating and/or lowers the auto igntion point of the fuel.
2EHN (2-Ethyl-Hexyl Nitrate) is what the petrol companies use to improve Cetane, but it can be bought from various suppliers by the litre or more quite cheaply. (Trinity Chemicals- "Biotane", Ebay etc)
Just add it 1000:1 ratio to make your own Shell V Power on the cheap.
Some diesel additives will contain very small amounts of 2 EHN, but hardly enough to make a difference overall, basically they are a mix of a tiny amount of 2EHN, a perfume and a carrier (distilled oil).
A lot of research was done by a German manufacturer in regards to additives and the results are out on the net somewhere.
Seems a worthwhile addition is 2 storke oil, the stuff you mix into the petrol of scooters and lawn mowers.
It has a lower auto igntion point and seems to burn better when compressed (auto ignite) that it does when you set fire to it and it's auto ignition point is lower than pump diesel.
So it will ignite completely at a lower pressure in the cylinders than diesel, brings down the AI point of the diesel it's mixed with.
It also has amazing lubrication propierties as it's designed for very tiny amounts to lube bearings in fast running 2 stroke engines.
Adding 250ml to a tank of diesel really does make it a lot smoother.
I've used both in various diesels, my Jag X Type loves the 2 stroke oil and a splash of 2EHN, it ran almost as smooth as a pertol.
You'll find 2EHN is in a lot of Biodiesel production, both commercial and home brew to counter the lower calorific value of certain vegetable oils, so you may find it forsale there too.