I purchased the AOG 30 in drop in Grill and am waiting for shipment. In the meantime I am going to start building that section of the kitchen. The cutout height is listed at 8.5 inches. My first question is do I hardibacker the area the the bottom if the grill would sit above of leave that open to the cabinet? If I am supposed to place hardibacker directly below the bottom of the grill I would assume the 8.5 inches would mean the the bottom is sitting on the hardibacker?

I did not cover the bottom of my grill cut-out. It is open to the cabinet. You may want to until you have the appliance, if you haven't already started. I have found that, although the listed dimensions would have worked, I tweaked them in my set-up to fit better.