MSP430FR2311: ADC channel 12 (temperature sensor) only characterized in datasheet at 3V ...wondering what accuracy I would get at 2V?

Table 5-22 of the datasheet is what I am referring to....

I suspect that it depends mostly on the internal reference. 

There is some calibration data in the TLV structure.

Did you see note 2? You will need to set a lower reference voltage anyway.