In this paper we investigate the multi-user SIMO multiple access channel. We derive the functional describing the achievable sum rate in a given channel. The mathematical behavior with and without successive interference cancellation is discussed. Furthermore we show that a practical rate prediction based on the well known SNR gap concept might be misleading, in some cases. Finally we show that the optimal power allocation which yields the highest sum rate is well suitable as a starting point for bitloading with discrete modulation alphabets.