Actually, according to Kodak, and testing prior to its release by Bruce Fraser, its OK but not ideal to use 8-bit ProPhoto. IOW, if you have an option, use high bit. If you have ProPhoto RGB 8-bit, its not the end of the world. I kind of doubt Joe's space would be that much different (I don't know what the encoding TRC gamma is) but I think its a pretty wide gamut space. Since I know both Kodak and Bruce did test 8-bit ProPhoto and said its useable, I'd stick with that but ideally, you'd set the scanner for high bit capture.
Converting from 8-bit ProPhoto to high bit buys nothing here.