Do you not think that the precise tonal curve used to map the Alexa image into a viewable space is being used to calculate the ISO800 base? I can guarantee that the raw linear light sensor data is way too dark lit ISO800 to be of any use at all. A development curve is needed. Same with the RED - use REDGamma. With the FLUT system, it means that the ISO rating set in camera, and when viewed with REDGamma will give you a mid grey brightness that is correct when you measure the scene with your meter and set the lens and camera accordingly.
If you're talking about a sensor - they have one fixed sensitivity, and it's down to how the pixels light gathering is designed and the filters that come before it in the optical chain.