CSMP math: lots of higher level stuff, but very little writing. Last week, we worked on multiplying fractions, and it is only a 1st grade curriculum!
BFSU science: Very meaty stuff, but designed for this age range. So the activities are geared toward children that have limited writing skills and attention spans. If you need more challenge, you can beef it up with the optional writing and reading assignments. It is teacher intensive, but that lets you have control over how challenging to make it.
FIAR: We did this at 4.5yo, and it was wonderful. I added challenge by doing a vocabulary study with each book. It could work for a 5.5yo accellerated child, but you may end up having to do more prep work to make sure you have enough material. And there will be times that you will just have to skip a day, because there is not enough material there. (For example, there is often only one math lesson, and it is usually about counting. Those days, I either doubled up on the other topics or skipped the day altogether.)
We started TOG at 5.5yo, and it has been a wonderful investment for my accellerated, history-loving dd.