But even if that little stub wing would solve the retreating blade stall problem, what about the advancing blade approaching transonic speeds? Huge amounts of drag will be produced there.
I've never understood this whole obsession with getting choppers to go so fast. It's not what it was designed for. And if you do have a need for both high speed and vertical take-off, then you can always put your name on the list for a BA609 Tiltrotor.