Sorry to resurrect this post.  I've just bought a copy of this disc and heard it once through.  I'm glad to report that this is without doubt a great Mahler 6th.  Granitic, serious and trenchant, if Klemperer had done a Mahler 6th, I think it would have sounded like this.  The orchestra plays terrific throughout (yes, minor slips in the brass; no, nothing major and certainly forgivable in a one-take live performance), and the percussion is fantastic (the tam-tam player is so loud in the finale he almost covers the second hammer--he deserves a raise!).  Speaking of the percussion, the rute in the middle of the finale (accompanying the furious fast string-led march) is replaced with a snare drum for some reason.  Sound is crystal clear but extremely dry, which renders the string sound somewhat wiry.
About the tempo: besides the Andante Moderato, which is probably the fastest around at 12:12, the other movements are pretty normal in tempo, and the first movement is on the slow side.  By taking the Andante Moderato at such a speed Barshai seems to emphasize this symphony's lineage from Beethoven and even Haydn or Mozart:  very flowing and light, but never lacking in angst in the climax or bitter-sweetness in the phrasing.  One of my favorite renditions of this movement.