dais:

HLOP in a VNA set up affords you aisles of 1.2m to 1.5m therefore you gain more storage and you can access more SKUs/pick faces. That is a big advantage of the HLOP as opposed to mid/low level pickers. In some warehouses this overriding factor is enough to drive warehouse managers to go in that direction because the compromise is you will need a new warehouse extension or off-site storage.

However your throughput suffers because you are going higher up. So it is a compromise. What can you live with and what can you give away.
Also in a VNA you will not be able to use a forklift or reach truck if you want to putaway full pallets. You will probably need a turret truck.