Correct me if I am wrong, I am not expert on the differences between both modes, but Direct Mode is just a way to avoid the problems that different Hz in monitor produce, isnt it?
So Direct Mode just will bring a bit better framerate perception, perhaps similar to work in extended but with the main monitor disabled, and a huge improvement in performanece in the Rift only would be possible by optimizations in the rendering pipeline in the game-engine of Frontier.
Anybody knows if FD has talked about this type of optimizations for improving the Rift experience?