With 20Mbits you would get only 1bit resolution, and since we're talking about analog data that's not enough. If we went to 12bit ADC we would need 320Mbits/sec.
You make a good point about needing more bits of resolution, but that's only a factor of ~10. I admit that I don't know the limits of the current sensors and rest of the system, but my guess is that they are not at the point where they are maxing out the bandwidth.
I expect that a full-bandwidth continuous surface-level brain scan would exceed traditional bandwidth.
