The cars coming down Manor Mount start and end in the same location as if they took the South Circular. You are not taking traffic off the South Circular are you? It's not that hard to understand.
It flows even worse because instead of having a constantly flow on the south circular, you will have cars trying to re-join the south circular off Manor Mount/Davids. Causijg the flow to actually slow down as people try and get back on it.