"But you have to put it into perspective that the 5.1 mix takes up only about, say 10%, of the total disc space. So in reality there's not much gain."
Yes. 1.3 Gbs, versus 380 Mbs. And believe me, the video was maxed out as much as possible. I had less than 1% remaining on the DVD (I had to recompile a few times to get it perfect - which is tough, since it takes 20+ hours to compile each time.)