Scott Simmons from the Edit Blog at the Prod Video Coalition has a must have article on Media Composer 6. It is a must read for some of it’s gotchas on ProRES.
While it can fast import ProRES footage like it can with DNxHD footage, it cannot then Consolidate ProRES footage, which means copying without recompressing, it can still only do that with Avid compressed footage. You have to Transcode ProRES footage, so it adds another layer of compression into the mix.