I have forwarded your question to the AMD development team for math libraries. Awaiting their response.
Just out of curiosity, these limits seem to be too big for all practical purposes. When could these be violated and bigger limits be required?
I have confirmed it with our math library team. This limit is artificially imposed because of the size of some internal registers.
We would appreciate if you can point out some applications that require an FFT of size bigger than the current limit, and where this limit can hurt.