build their kernels configured for 64K page sizes [...] Presumably they do this for performance on large systems, at the cost of less efficient memory usage.

My guess is that they do that to enable a larger amount of memory (very relevant on servers, which are RedHat's main target market). AFAIK, both the LPA (large physical address) and LVA (large virtual address) extensions, which allow using 52 bits instead of 48 bits for the memory address, require using the 64K page size.