Looking at this thing, it won't matter if you added any more cubes per side. None of the corners will ever move. It's an even worse case than the middle cube on a Triamese (or whatever that's called). All three axis are eliminated for each of LR,FB,UD.
Any extra cubes you added to each side would be able to function like the middle cube on the Triamese (center slices only).
And I can't imagine how the heck it could turn inside out. It would have to be made of rubber!
Still a great idea. Even better now than I understand it!