Stating the obvious:
CWL will not prevent the 'leakage' of pax to Bristol. Of (say) 200 pax on a Bristol flight, 40 may be from Cardiff's catchment area. That's not enough to fill an aircraft economically, so they will continue to go to BRS.
As to transferring the whole flight to CWL, that's pretty unlikely. The 160 pax from the Bristol catchment won't necessarily go to CWL, as some of the will be nearer EXT, BHX or LHR.
CWL will at best provide a departure point for popular destinations, well-served from BRS such as PMI, AGP, FAO etc, where volumes are sufficient for the aircraft to be filled from Welsh demand.