the throughput saturation value is given by the maximum spectral efficiency, obtained with CQI 15, on two spatial streams (or layers). This saturation value therefore does not change with the number of eNodeB antennas as long as the number of layers is fixed.
When the number of receive antennas at the eNodeB is increased to a number that is higher than the number of spatial streams, receive diversity comes into play. It means that the received signals are now exploited not only to separate the spatial streams but also to exploit diversity. Due to the feedback algorithm this receive side diversity than translates to an SNR gain. One can say, that due to the use of more antennas at the receiver, the transmission is improved in the sense that a certain CQI is utilized already at lower SNR compared to a situation where there are fewer antennas at the receiver.