Why do I only see the Run-Time configurable option for an 8 point FFT when I re-customize the core?
This is a bug in the core GUI interface. The run-time configurable option, should never be available for an 8 point FFT, as it is not necessary. This issue has been resolved in the Fast Fourier Transform (FFT) v4.0.
See (Xilinx Answer 29209) for a detailed list of LogiCORE Fast Fourier Transform (FFT) Release Notes and Known Issues.