NUI data is 16 bits with 3 reserved bits for player masks.
I.e., the conversion looks like: (((unsigned short*)depthPixels)[(((i * depthWidth)+j))] >> 3)
Which means that if you want to destroy your data and convert to 8 bit, that you need to scale by 2^13-1 instead of 2^8-1.
awfulDepth = (((kinectDepth >> 3) / 2^13-1)) * (2^8-1).
Why on earth you would want to do this is beyond me, but good luck.