I've come across mushrooms in powder form (sometimes even in capsules) plenty of times. There is theoretically a possibility of a decrease in potency, actually, because the powder form means that there is more surface area and the psilocybin contained in the mushrooms can oxidize and break down faster. However, you should not take a higher dose because you assume that this is the case. Powdered mushrooms can definitely be just as powerful by weight (or even more, depending on growing technique, species, etc.) as whole mushrooms ... although if you took a batch of whole dried mushrooms and ground them up, you wouldn't magically increase their potency, and if you ground them up and let them sit for a while, you'd actually decrease it.
To be honest I'm not even sure if there's any truth to the claim that caps have more psilocybin in them than stems, but I might be wrong about that. If there is a difference, it's not a very big one.
When I first saw this thread I thought it was about putting mushrooms in a blender with fruit and yogurt and eating them that way! I've done this before and it was awesome. If you do end up eating this mushroom powder, a smoothie or a brewed tea is probably going to be a better way to eat them than just choking down a pile of dry mushroom flakes (ugh).
