SNR for the most part correlates pretty closely to max MCS. However, it would be very difficult to average as clients are not always sending MCS-graded packets, basic frames, multicast, etc would be at slower legacy rates. it would be pretty difficult to even average, SNR is FAR more consistent (though yes it varies a few dB from moment to moment, but MCS can be present then legacy rates then back to MCS, etc, and you cannot average out that without is severely skewing the mean).