Sorry if you have already finished this question, I have just joined ResearcGate.
I did a quick serach on Google scholar, using the search term "Foot and ankle outcome score reliability", and found a lot of papers on the validity and reliability of the FAOS, in good-quality journals. It might be worth your time to do this search and select papers that are specific to your question.
EFAS just introduced and published a new score recently that is straight-forward to use and has been validated for several languages (Foot & Ankle Surgery 2018). You might want to check it out, first author is Martinus Richter...