Will 1080p RAW be possible?
Not using an external recorder. Even if the 550D outputs 1080p from the HDMI it would be upscaled and it wouldn't be RAW.
In RAW video mode in ML it's technically already possible. In crop mode one can record RAW at 2512 by 1080 px.
However the card can't write quick enough so you will probably only get a few frames of recording.
New advances may change this. The camera records 14bit raw, if a conversion in camera can change this to 12bit, then it will be able to record much faster.
But that may or may not be able to be done, so basically a waiting game.