P.S. I am unclear what the benefit of forcing the number of slots to be a prime number.
If (and it is a big if) the lower order part of the hash value is truly evenly distributed, then modulus anything should also be evenly distributed.
Of course, that "if" may well not be true.
However again I would argue that we should not be bothering with changing the slots number unless we can demonstrate significant real-world performance improvements.