Hi James,
The relationship appears to be linear over temperature, but we don't have enough data with respect to noise over the full-input range to make a solid claim. There are a couple of interactions to consider. From characterization data we see that the input referred noise level decreases as the reference voltage decreases. It is the percentage of full-scale that increases as the reference decreases. In other words, a smaller reference voltage has fewer bits of resolution. This may not be a big factor as the actual LSB size is smaller for the smaller reference.
The trade-off takes place relative to available gain. The input referred noise lowers with increasing applied PGA gain. This is similar to what I explained above where the number of bits of resolution decreases but the LSB size is smaller.
Best regards,
Bob B