There is no advantage to setting the FIFO depth <16 because the same number of LUTs are used for FIFO less than 16. The documentation and MPD will be fixed to only allow the following sizes:
RANGE = (0,16:C_ASYNC_CLKS*128 | !C_ASYNC_CLKS*8192)
The data sheet and MPD file will be updated in a future release to indicate support of a minimum FIFO size of 16. Currently, the minimum FIFO size supported is 16 (i.e. PARAMETER C_FSL_DEPTH = 16).