so far BD shows best quality of audio signal.
5.1 stream is a typical DSD format with limitation to 22KHz but filled up with noise above 22KHz.
so they used same master as for SACD just converted it to PCM.
Which likely makes no audible difference whatsoever.
However, what I see that WOULD make a difference is there is signficcant EQ difference between some channels of the BD 4.0 -- the 'blue' channels are treble boosted versus the 'red' channels -- moreso than on the 5.1 and on the 'bootleg' 4.1. So I definitely would expect the two 4.x masterings to sound different, even aside from the presence of .1 content in the 'bootleg'.
i cannot agree with your first sentence. albeit range above 20KHz considered to be not audible for humans,
in fact it does enriches audible range with harmonic microdistortions, which add liveliness into the sound.
that's why people with sensitive hearing can easily hear the difference in the same sound but in different
freq. range. actually this is main reason why vinyl despite all its shortcomings, still hold its position as an
audiophile's media. the needle, during sliding on vinyl surface, create lots of harmonics which add specific
charm to vinyl's sound.
as for mastering or remastering of the Parson's stream on BD, i can't tell anything as i haven't heard it to
compare to bootleg. spikes of freq. on screen can be only reflection of the particular place of the stream on
which was placed cursor during screenshot.