Why does Equation 3 in the data sheet quote a dynamic range of -2^(n-2) to +2^(n-2) -1 for the symmetrical case?
This equation is incorrect. You must have 1 extra bit if you want a truly symmetrical signal.
The correct symmetrical equation is:
-2^(n-2) to +2^(n-2)
For a detailed list of LogiCORE Sine-Cosine Look-Up Table (Sin Cos LUT) Release Notes and Known Issues, see (Xilinx Answer 30162).