The problem is that too many variables exist to definitively say this PC will make this system sound better in this way or another system will sound about the same with or without the PC. These manufacturers are making a product that reduces line noise. If it makes the audio system sound better, that's almost a side effect.
Published measurement data of these PCs might be one way to show that they are working but how they affect the sound of an audio system is almost incidental. For example, plugging an amp into one could make the amp sound worse because it limits current. Measurements can only go so far.
Supposing you bought two different cakes from two different bakeries. You take those cakes to a chemist and have him analyze them. He gives you a list of ingredients and relative weight of each ingredient. That analysis might give you some idea of the flavor of those cakes but until you take a bite, you will not know how they taste. Furthermore, the chemist cannot tell you how each cake was mixed and baked- the process. And the mixing/baking process could have a significant impact on the flavor.