Anyone sucessfully found a way to mix the two resolutions without having to render?

I have a project shot on 7d/5d at 1920x1080, converted to cineform 1920x1080
Also shot on si2k 2048x1152.

I have not been able to figure out a way to mix the footage on the same timeline, without render bars, or sluggish playback on one of the resolutions. Trying to avoid having to re-render all the footage to a higher resolution. (don't think it is possible to transcode the 7d/5d to 2k res).

I have been given some suggestions, just seeing if anyone else has found a workaround.

help!