I don't see how it is so controversial to think of being able to see the typically unseen faces as being a significant variation from the existing puzzle as it was when the WCA was formed. Say you could have a mirror behind the cube, so you could see almost every piece just by moving your eyes. The way you'd 'think' about the puzzle would be different, you'd focus on different things, there would be more emphasis on mentally connecting the two images of the cube, less emphasis on tracking pieces in your mind, less emphasis on deducing the 3rd unseen face of a corner, instead of concentrating on 2side pll recog it would be easier to spot key PLL patterns. The actual mental process of solving would be different, enough that it's essentially a different puzzle.
Now, I know the mental process is different for different methods in a standard solve, however there is nothing to stop a Fridrich solver learning Roux, but there would be something to stop a standard solver from getting key information about the cube that a mirror solver would have, the only way they could evenly compete would be for everyone to use mirrors.
There isn't any proof that being able to see the unseen faces without rotating the cube would make you faster, because I presume nobody is practicing it as a skill, however it is an extra option that someone with a stickerless cube could use, that wouldn't be available to someone without a stickerless cube.