Random unitary beamforming (RUB) is an attractive low-complexity transmission scheme for multiuser MIMO broadcast channel. When there are a moderate number of users in the system, the sum rate performance of this signaling scheme becomes mainly interference-limited over high signal-to-noise ratio (SNR) region. In this paper, we study the performance of RUB-based multiuser MIMO systems when optimum combining is employed at mulitple-antenna mobile receivers to suppress multiuser interference. We adopt a geometric approach to derive the statistics of the output signal-to-interference ratio (SIR) of the optimum combiner, based on which a sum rate upper bound is obtained over high SNR region. We show through selected numerical examples that the proposed RUB with optimal combining achieves much better sum rate performance while maintaining low feedback load.