what appropriate "Allocation Unit Size" for an exFAT SD card with ReadyBoost
I would use exFAT, it has slightly bit better performance than the others and will also allow you to use more then 4 GB of space for ReadyBoost.
For the allocation unit size, I belive 4 kB will work the best but I could be wrong. This is the size of the chuncks that ReadyBoost uses and it will give you the best performance for ReadyBoost.