From the research that I have done on diesel fuel additives there are several reasons to use them. It can be used as a biocide, water in fuel control, a stabilizer, a lubricity improver, an injector cleaner, an antigel, or a cetane improver.
The diesel fuel additive that you chose, if you use one, depends on what you are trying to accomplish with the additive. If you are using an additive for fuel economy improvement only, there is no clear cut answer.
Most of the independent studies that I have read indicate that an additive that is designed as a cetane and lubricity improver, and an injector cleaner, will improve your fuel economy. Although the economics of using an additive usually works out to the amount money that is saved on fuel is offset by the cost of the additive.
So you probably will not save money on your fuel budget, but there are other benefits that vary by the properties of the fuel that you are using.
As for as using an additive in a 400 HP C9, but not a 425 HP C9, there is no technical evidence to back up that viewpoint that I have ever seen.
Gerald