Unfortunately, fuel additives will provide no benefits
Not true in the case of diesel. Cetane improver improves the cetane number, which makes it burn more quickly and more completely. Only up to a point though - better quality fuel has a high cetane number already, so the additive won't do as much. It has more of an effect the worse the fuel is. Essential if you are thinning veg oil with petrol. Also note that diesel is worse in winter - the anti-freeze additives in it lower the cetane number, so you will get better effect in winter.
It may depend on the engine a bit too - the ECU may vary the injection timing with different fuel characteristics, but not too sure about that.