I can't really answer your question... but I can give you some information that might be helpful to drawing your own conclusions.
I use a 5D IV and 5D III and I don't shoot video... so I'm not speaking from experience with 6D video capabilities, but I DO shoot astrophotography and the balance between high ISO vs. noise and dynamic range are a big deal (fortunately in astrophotography we have special techniques to deal with noise because we get to take many pictures of the identical subject knowing that absolutely nothing "real" actually moved between shots and anything that did move or shows up randomly from frame to frame doesn't belong in the picture.)
As you increase ISO, you'll get more noise. But it turns out Canon uses two different techniques to boost gain when you increase ISO. At the low end of the ISO range, increasing ISO causes the camera to perform "upstream" amplification. This happens when the information is still analog (before it goes through analog to digital conversion (ADC)).
Then at some point, it reaches the practical limits of what you can expect from analog gain, and switches to digital amplification (simple multiplication of digital values). This is known as "downstream" amplification.
When you increase ISO, you lose some dynamic range. But it turns out when you increase ISO via straight multplication, you lose a stop of DR per stop of ISO. There are some techniques to alter how the multiplication is done to try to protect some DR. Basically when the camera is primarily using "upstream" amplification the result is a tiny loss of DR. For "downstream" amplification it's a more significant loss of DR. The inflection point where the graph bends shows where the camera is transitioning from one technique to the other.
For YOUR camera, here's the graph:
Looking at the graph, you can see that once you get beyond ISO 1600, the camera begins to take a near-lineaar loss of dynamic range for each stop of ISO. But below 1600... it's non-linear and you don't lose very much DR for each stop of ISO.
So based on this data, if you can accept the level of noise you get at ISO 1600, then that's the point where you maximize high ISO while also protecting most of your dynamic range.
The 6D seems to be very comparable in sensor performance to a 5D III ... and I do own a 5D III (actually the 6D may be just fractionally better). With the 5D III, I would use ISO 3200 without even thinking about it... and even ISO 6400 had noise, but I could easily process that in post. But I do still photography - not video. In still photography I'm able to use de-noising tools which allow me to tune how aggressive the noise reduction is based on tonality (more aggressive in shadows, less agressive in highlights -- which tends to be where more detail is anyway and typically will have less noise). There are also ways to generate a mask that selects edges of contrast & detail... this allows you to avoid de-noising (which results in softening and loss of detail) in areas where you want to retain the detail. These take a bit more time, but give you more selective control than just a global de-noising adjustment that treats every pixel the same.
I'm not familiar with video editing so I'm not sure how that works when you attempt to do post processing of video.
Best Regards,
Tim
