new Pentax 645D medium format with 50mp Sony CMOS sensor with video recording mode ?

[Kolor-Pikker]

Off the top of my head, is it possible that in stills mode the limiting speed factor may not be sensor read, but debayering, writing to card etc etc? No idea if that is the case, but extrapolating just from still burst speed may turn out not to be an accurate way to predict video characteristics?

Absolutely. The Canon 5D3 for instance manages a 6fps stills rate with its Digic processor, while the 1Dx needs two working in tandem for 12fps, even with the sensor sporting less resolution.
Another aspect is mechanical speed, each time a stills camera takes a shot it needs to close down the iris, flip up the mirror, actuate the shutter, and then flip the mirror back down fast enough. In the case of the 1Dx, the whole process needs to happen in 83.3ms to support a 12fps frame rate, but actually happens in 60ms, so that you actually have time to see an image and for the AF sensor to regain focus tracking.