(Well, the comic in question specifies 100 trillion, not 8.) Like I said, all food would have to be produced locally from recycled waste. I'm not sure about the heating problem, but I'm assuming direct chemical manufacture of protein/vitamins, which, if possible, should cut down on the energy requirements. Given adequate surface ventilation, it might be possible.
Overheating would definitely be a problem for a place like necromunda/terra, unless they have some very fancy future tech for efficiently pumping waste heat into space. *shrugs*. I dunno. Maybe they couldn't exist. But my point is that, if hive or forge worlds did exist, in anything resembling the official description, they would necessarily house huge populations.
EDIT: Tokyo comparisons don't really cut it, since none of our modern cities approach the density of an arcology.