Customers have asked when AME will support 64K medium pages. The answer is that they already are supported, but disabled by default via the vmo vmm_mpsize_support tunable (-1). 64K pages can be re-enabled via this tunable and a bosboot/reboot.
The primary reason they are disabled is that development performed tests with AME & 64K on various benchmarks and determined that the additional overhead of decompressing 64K pages (16 4K pages) outweighed any benefit of medium page use.
As a result, AIX development does not recommend mixing the two technologies, but customers are free to experiment.