Tom,
The problem is the down force of the thrust the hex creates. I flew a 16,000 mah under the belly with Velcro and got a 34 min flight with the first warning at 27 min. This was on an lihv with no camera. I designed the whole setup to put the camera and all and the motors got really strained with only yielding a 24 min flight on the same battery.
I think a dual setup of 10,000 to 12,000 mah total would yield a nice flight time of 30 min without the ESC and motors melting down. Everything I have developed with the help if the community is tested a lot before I sell it. I don't like making something that can potentially ruin someone's H or void the warranty.