Also, multiple audio streams shouldn't affect this since you only play one stream at a time.
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A much better way to address this long term would be for players to read at greater than the 1x speed (36 mbps) so that they could fully utilize the full BD AV bandwidth of 48 mbps....
Multiple audio streams do have an effect, because they are multiplexed in together. The player has to read the container at the mux rate and then extract the parts which it currently needs, ignoring the rest. It's done this way so A/V sync is easy to maintain and there's no seeking required to pull the video from one location on the disc and the audio from another.
Also, no one is going to use less than 20 bits per sample for a feature presentation. With the way PCM is stored that still takes 3 bytes (24-bits). Compression (both lossy and lossless) can pack multiple samples together to recover the wasted 4 bits.
The 48 Mbps allows for buffered data to burst above the 36 Mbps transfer rate of the drive. But the long term average bitrate can never exceed 36 Mbps without creating a new non-backward compatible format, as all the old players with the 1x drives couldn't play the disc correctly.
