The thing about a rangefinder is that you MUST learn to see the world as it will see it.
You can't get lost inside the SLR kaleidoscope.
The reason a Rangefinder is good in the street is that the photographer learns to SEE what it will see.
Its like learning to play a violin. Following the SLRs are better argument, guitars must be better than fiddles because they have frets and you know where all the notes are.
So, it isn't so much whether a RFDR is sharper without the mirror, it is that WE become sharper when our eyes can see.