For simplicity and perlness points, an extra ++ to this one... :-)

Running haukex's test-and-benchmark with AM's ltr() included (without 285075=>'PERL' in the test list), ltr() appears about 10x more efficient than colname(). A four-Z-based ltr(), however, is about 75% less efficient than colname().

Excel 2007 upped the limit from 2-letter/256-column to 3-letter/16384-column, and that hasn't changed as of Excel 2016, so the more-efficient ZZZ-based ltr() is sufficient for now. :-)