Yes, sadly this method will only get an extra 5 or so bits of range from the 2nd iteration with such a large divisor as 256000000.
If you want to divide by exactly that then it’s better to divide by 15625 (5^6) and then shift the answer right by 14 bits (2^6 * 2^8). That will let you handle clock cycles up to 102 days.
Thanks all for the feedback on these issues. There is a PR now for the next release 1.0.2 that will address all of these issues (and a few more, like @dagema’s issues with the Arty board) : https://github.com/sifive/cinco/pull/19. Will post on the forums when the update is available through the Arduino IDE.