I would guess that the 7D2 video performance will be limited by the same overheating problems that limit all DSLRs. DSLRs are camera that are designed to take photographs, but happen to have some ability to make video. The video performance has to be limited to prevent the sensor and electronics from overheating and failing prematurely.
I don't think sensor heat has anything to do with it. You don't need to expose the sensor more times, you just need to throw away less data.
I believe that it must have something to do with it. The 1DC has a heat spreader around the sensor to keep it cool. The Sony SLT cameras are known to overheat while shooting 1080p60 video. They don't actually overheat, but first display a warning, then shut down before any damage can occur. Even my RX100 will shut down if I shoot 1080p60 long enough at a high ISO. I'd imagine the thermal output increases as the sensor size increases. The GoPro Hero and most handheld video cameras have tiny sensors.