To get good high ISO you want to keep as much light as possible, that is ideally the camera would be monochrome.
Color separation is a complex area though. A good basis for good color separation is a low noise sensor, then you can separate colors even if the channels differ very little. That is you can have wide overlaps and let through lot of light in all filters and still be able to separate colors.
With a noisy sensor it's better to have more difference between color filters so the channels differ more (you get more saturated colors natively). In extreme cases you don't have overlap at all which I think was the case for some older prismatic TV cameras in the analog days. This indeed gives very sharp separation, but due to the lack of overlap there are colors that will be registered to be the same that would be seen as different with overlapping filters.
All(?) modern sensors today have wide overlaps and and quite low saturation colors natively. This allows for registering lots of colors, and improves ISO as more photons are captured, but also put requirement on the sensor to be low noise so we can through the color profile add back saturation and visibly separate colors.
When working with DCamProf I noted that many cameras, specifically Sony, had a high sensitivity on the blue channel. This made it very complicated to make realistic blue tones in daylight (daylight has high blue content in itself). The solution is to render blues lighter than realistic, which indeed is a popular "look" which many color profiles implement in way way or another, regardless if the sensor requires it or not. I felt that Canon's color filters where better balanced to produce realistic colors though.
The DxOmark SMI index is a measure how well the ColorChecker 24 color can be matched with a matrix-only profile. A disadvantage of that test is that it doesn't say anything on how the cameras react to "extreme" colors (very high saturation colors), so the Sonys fair very well (much thanks to their low noise) although I personally prefer the response I've seen in Canon cameras as their easier to manage in the extreme range.
Are there tradeoffs ISO vs color separation? Maybe/probably, but I don't really know how they are done. A guess is that the high blue channel sensitivity in the Sony is a high ISO thing, plus that they realized that most subjectively like lighter-than-realistic deep blues anyway. That doesn't really hurt color separation though, just the possibility to make realistic colors in cool light. You could make the color filters be more similar in the overlaps to let through more light, and increase the similarity more than what is suitable for an ideal color separation tradeoff.
With DCamProf I made a color separation diagram which can be used to make comparisons how well a camera can separate colors compared to the eye:
http://www.ludd.ltu.se/~torger/dcamprof.html#ssf_csepHowever one needs to factor in the sensor noise to make really good sense of that.
If we talk about the "medium format magic colors" or any other camera's supposedly magic color I'd say that it's almost only about well-designed subjective color profiles that fits the taste of their users. Any sensor today of reasonable size in good lighting condition have sufficient color separation to design almost any type of look you want, at least that's the indications I've got from my work, so for any ISO sacrifice made it doesn't seem to be large. Going from the camera's raw colors to a robust color profile is not an easy thing though. One may think it's only about shooting a chart and matching, which in the case of reproduction work it may be, but for all-around photography you need to apply contrast, a "film curve", and that will modulate colors as a side effect (as our brain's color perception is tightly linked to contrast). That's why matrix profiles designed for a linear curve works so bad with contrast applied, oh well bad if you ask me, some like the result anyway and it's much about taste.
A well designed color profile is designed to match the film curve it's intended to be used with. There are really no established color science models to do this so it's up to the profile designer to invent their own models, and the manufacturers have done so, and so have I with DCamProf. We all end up with different results of course, as we have different design targets and tastes.