As a previous RED owner, the Original "M" sensor was Native 320 ISO. The "MX" Sensor is Native 800 ISO.
RED cameras do not like you to change ISO below the Native rating. I can attest that the image starts getting crunched and less "filmic." I hate having to put 13 stops of ND and a Hot Mirror (required for all RED shots) out there just to take a daylight shot at f4 (no joke!). A serious flaw in the camera's design IMHO.
My experience so far with the 7D in RAW, the basic quality of the image doesn't seem to get wrecked if you vary the ISO up or down (within reason, of course). I've done critical tests between 100 ISO and 3200 ISO and find staying within 100 ISO to 320 ISO is great.
Well RED MX, and all native 800 iso (like alexa, bmcc, c300) sensors tend to lose DR in highlights when changing iso to lower than 800 and in shadows in higher than 800.
Then you get not so nice rolloff in highlights and really messy noise in shadows. So in bright daylight you have to use heavy IR ND's.
All filmmakers want high native iso, because with lower iso rated sensors you need lots of lights on set to get good base exposure and then even more for lighting the actors to get contrast and separation from the background.
Now indoors with 800 iso (or ASA I should say) you can use more natural lighting without introducing lots of noise to the image, and even "light" actors faces with bounce boards etc.
So once again, higher iso rated sensor = more money saved for producers, and low budget filmmakers.
7d sensor is 100-200 native so outdoors its all great, but indoors it can get messy. Even film stocks got to the point when you use 200 for outdoors and 500 for indoors.