[Daniel Neutzsky-Wulff] "The reason for asking is that I want to maintain all the dynamic range of the original footage for color grading."
h264 is 8b/420 and prores is 10b/422.
You won't lose any dinamyc range when converting.
I would convert the H264 to Prores/Prores HQ (if possible with 5DtoRGB), and forget about H264 (not supported by FC neither by Color), and forget about proxys. TIFF and JPEG have nothing to do here.
If you insist in working with proxys, don't forget that the only way to reconnect to the original h264 files is using L&T and the Canon plugin.
rafael
http://www.nagavideo.com