I use stanadyne for 2 reasons. Lubricity and the cetane improvement. The bottle says 8oz of stanadyne for a full tank. Im assuming that means that the 8 oz gives it the full 5 point cetane increase.
Is the cetane increase additive ? In other words, if 8 oz increases cetane by 5 points, does 16 oz increase it by 10 ? And if so, is there yet another noticeable power increase ?
Actually, about the most you can raise the Cetane of diesel is about 7 points.
You really don't want it to high because it will cause more problems than it solves.
A cetane number which is too high may cause too short an ignition delay period. This changes the timing of the pressure peak, resulting in loss of power. When this happens, many of the performance problems associated with low cetane fuel will result. While the problems due to low cetane largely disappear after the engine warms up, with too high a cetane, these problems will persist even with a hot engine.
There is not a noticeable power increase. There will be increased efficiciency if you have poor fuel (low Cetane) and you increase the Cetane number to a that of average fuel. The engine may run smoother but only if you started with poor fuel and brought it up to average or better.
Cetane is really a measure of how well the fuel combusts. As long as the fuel you get meets the minimum requirements of the engine manufacturer, a Cetane booster is not going to show great results. Plus, once the engine is up to temperature, even poor fuel burns pretty good.
The main reason to use Stanadyne is the lubricity and the fact that it really protects the pumps, especially VP-44s against damage in case water makes it past the filter and into the pump. Stanadyne makes good product. But it's not a "feel it in the seat of your pants" product unless you have poor fuel and the truck is running rough as a result.
Attention: TDR Forum Junkies 
