This paper addresses the application of filter bank multicarrier (FBMC) systems to multiple-input-multiple-output (MIMO) channels. In particular, it is investigated the FBMC modulation based on OQAM, known as FBMC/OQAM. Existing solutions reveal that FBMC/OQAM remains competitive with the orthogonal frequency division multiplexing (OFDM) technique when the number of streams (S), transmit antennas (NT) and receive antennas (NR) are related as follows: S = min(NT,NR). State-of-the-art techniques fail to achieve satisfactory results in FBMC/OQAM systems when S < min(NT,NR). That is because the strongest spatial subchannels exhibit higher gains in MIMO-OFDM than in existing MIMO-FBMC/OQAM systems. To determine if FBMC/OQAM can be successfully applied when strict inequality holds, this paper provides experimental performance bounds, which can be assumed to approximate the optimal performance. In this sense, numerical results show that FBMC/OQAM is able to slightly outperform OFDM when S < min(NT,NR) by attenuating the residual interference 20 dB below the equalized noise. This insightful information could be used in the design process of future techniques to approach the experimental performance bounds.