while i haven't tried a 256gb sdxc i have used a 128 and it worked perfectly with the stock firmware, however i found that the adapter didn't work well with rockbox, batterylife went from 20hours to 8 :(.
i then bought a Komputerbay 128gb CFcard which would only boot once out of every 5 times. Just watch out for that as there's at least one other person with a similar Komputerbay CFcard booting issue. i guess the only thing it had going for it was that it was almost half the price of a more reputably branded one.
after a few years i gave up and bought a dx50. i really miss the form factor of the mini.. plus it was basically indestructible.