i've had this done several times, but my cobbler has only ever been able to shorten by about 1/4" to, at the most, 1/2" (a little less than that actually).
taking off any more throws off the balance. so, when i want to have it done, i take the shoe to the cobbler, and they will tell me how much they can take off of that particular shoe- sometimes it's a fraction of an inch, and sometimes they can't take off any.
so i wouldn't recommend buying shoes if you will ONLY wear them if a lot of the heel is chopped off. because there's a possibility you won't be able to get any taken off. i try to only buy shoes that i would still wear either way.