I haven’t rendered any projects in 16 bit. I did some basic trials with existing projects and with particle emitters.
The first thing I did when I got Motion 2 was I put a fog particle on the stage and played it. I then switched the project to 16 bit and the fog looked noticeably better, and it played at the same rate, real time, more or less. When I switched it to 32 bit float, the fog dropped down to seconds per frame.
My system is a dual 2.5 Ghz G5
OS X 10.4.1
nVidia 6800 Ultra 256 meg
4.5 gigs RAM