It remains to be seen what the high ISO noise levels are as we are dealing with a full frame sensor and we are in unknown territory with 50mp - I know that if I crop my FF cam (5D3) to the same FOV as a 1.6 cropper it is still a lot cleaner. The actual ISO range is fairly insignificant to me, If it is clean up to ISO 3200 in crop mode then that would do me fine as I rarely bother to shoot at higher than that (even with the 5D3). The whole point to considering a Camera like this for me is to use it as both a FF landscape/still image Camera and a crop Camera for reach limited situations and occasional wildlife. If it worked out OK I could sell my 5d3 - it may be a better option than using both a 5D3 and a 7d2. I could never give up a FF that's for sure. 5fps again would be fine for me as I do not do much bird snapping these days.
Of course, actually ISO is meaningless, only noise generated matters. The assumption I am working on is not one of sensor size, but pixel size. To date, full frames may have comparable MP counts, but larger pixels (less pixel density) which typically have responded better in terms of noise. Larger pixel = more ability to mitigate noise through design, due to size being less of a constraint in what you cram into it, is my assumption.
From that perspective, I am extrapolating based on the pixel density of the sensor, which by my logic (which could be complete crap), would predict similar pixel performance from similarly dense sensors. No full frame vs crop frame, just pixel density being the 'real' factor. What this means, generally, is if we applied the same sensor tech in a 1Dx (22MP? I think) to a APS-C with 8.5MP we would get the same results for noise performance (APS-C to FX is 2.6xz the area).
If I am missing a logic fault, I would be happy to know what it is. The pixel density is also the 'reach' factor we get, which in the case of this monster, is negligible (it would have the same reach as the current APS-C, practically)