in order to digest mushrooms, you have to break through the cell walls. this is generally done by cooking. but, guess what? b5 is destroyed by heat.
chopping might help a tad, but that's basically just chewing them.
so, if you want mushroom specifically for b5, you're probably out of luck, at any kind of reasonable serving size.
now, in theory, cooking can sometimes make fat soluble vitamins more bioavailable. but, mushrooms are very low in fat soluble vitamins, so cooking them isn't going to do anything but destroy the nutrient content that does exist. the one counter-example is probably choline.
conclusion: mushrooms are a good source of an unusual type of fibre, and not much else.