I fooled around with the 5.1 mix a little, but I ran into a pretty major obstacle - I don't have the NTSC GOUT to sync it to, only the PAL, and none of the speed conversions I've tried on video or audio are entirely successful (I suspect the NTSC and PAL versions don't have exactly the same content frame-wise).
Does anyone know a good place to get the R1s in the UK?