That's beyond my pay-grade technically. My guess is that whatever minuscule compression artifacts may arise from converting to prores would be dwarfed by a whole set of technical issues, such as motion judder or the aliasing issues of the 5d sensor. (I probably shouldn't tell you that regardless of the level of technical perfection you achieve, the projector or projectionist will destroy it at 98% of the venues it will ever play.)
That said, if you want to talk tech, you'll get a better response at some other boards than you will at d-word. Try these:
http://www.cinema5d.com/index.php
http://www.dvinfo.net/forum/
http://www.dvxuser.com/V6/
