MIMO can increase sensitivity by 3dB because MOMI use spread spectrum CDMA technologies. According to CDMA properties, if use two receiver antennas, the signals is doubled, but noise is still white noise, so sensitivty improves 3dB. If use four antennas, improves 6dB.
But if your signal is not CDMA, then can't get the improvement. Diversity technologies normally only select the better path of the two paths. There also may use combination of the two path signals, but it is not mature, and is hard to evaluate. Anyway, the best of diversity is 3dB.