If the 5.1 version is mixed and mastered better than the CD, it will still sound better when downmixed to stereo. This has nothing to do with number of speakers or surround effects, but with plain dynamic range.
I intend to try a downmixed version of the 5.1 mix for regular use. I did the same thing for Opeth's Still Life, where the CD mix is terrible and the surround mix of the special edition is great (seriously, it has made Still Life one of my favourite Opeth albums).
I can't foresee whether the 5.1 mix of DT12 is any better than the CD mix though, it's possible that both are equally brickwalled.