I have to share my experience with saxon here, for DD last year, we were planning to join her in a school which follows saxon math curriculum, school principal was boasting that Saxon was very dificult curriculum and lot of pupils need additional help at home. DD was 4 that time and we are looking for a grade skip. principal said no way she was going to skip on saxon curriculum and we had to join her in a different school anyway... But we started doing that at home just out of curiosity and within 1 year from Feb last year to may this year she completed first, 2nd and midway through 3rd. first half of a grade is basically 2nd half of previous grade, at least in Gr 1-3.
for gifted kids it is kind of very repetetive, DD had done a week worth of material several times over the weekends
just my experince,
