I've ground down a seatpost 0.2mm and it takes a LONG TIME-LOTTA WORK WITH A HAND HELD MECHANICAL SANDER. I think it took about 1 hour of grind, test, grind test etc, and it looked cheesy at the end. It was a long-maybe 350mm-seatpost, but I probably just did about 7"-175mm- of it.
Pay the extra $$
Charlie