This paper presents a queueing model that provides a performance upper-bound for Multi-rate Downlink SDMA systems. It is shown that the performance of Downlink SDMA systems depends to a great extent of parameters such as the number of active users, the traffic arrival process, the queue size, the number of antennas and the set of available transmission rates. Using this upper-bound as a reference, the situations in which a Downlink SDMA system is able to near attain the upper-bound performance are discussed.