so I went back over my calculations to double check everything and I realized I made a crucial error (well, 2 really).
First, I used an average GI of 50 to calculate FP, when in the book it clearly states, as you mentioned, that the GI is 81. This brought my FP down to 9.03 for 1/2 cup (the amount required for 16 brownies).
Second, I made an egregious error when it comes to baking: I didn’t use my scale! I assumed 1/2 cup of tapioca was 103g (based on the raw cassava values) but tapioca starch is extremely fluffy and light! 1/2 cup weighs only 63g! Using this information, the FP value using values from the book comes out to 10 FP per 1/2 cup of tapioca starch, extremely close to the 9.03 I calculated above using raw cassava.
Lesson of the day: when baking, never ever forget your scale!!