Some good info. on water pumps.
Over-all, the SL6 has a good pump and pump scroll design. If you look closely at the scroll, it is not perfectly round and the pump's impeller is off-set with-in the scroll's pocket area. There is increasing space between the impeller and the outer scroll wall as the coolant approaches the water jacket entrance.
As many of us know, the impeller rides close to the back wall of the cavity.
Last time I checked, there was about .06 of clearance, so a bent shaft, bent impeller blade or rust scale build-up can cause the impeller to rub on the back of the pocket. This close fit does make it hard to add a separate "disc" fastened right to the impeller blades. Again, base on the already close fit, my guess is that adding a plate would not show much gain.
I have always felt that the "pinch point", where the impeller passes the opening to the water jacket and moves back into the scroll pocket is most important to good flow and over-all pump performance. This is why I had to weld-up all the corrosion I found in Twiggy.
You can get a good look at this intersection if you cut a viewing window into a scrap pump and then mount it to your block.
Getting a real close "fly by" at this place increases the pressure in the water jacket. Knowing this is one reason why I don't like the gasket thickness worth of space between the flow deflection 'ramp' and the pump's separator plate. I think it would be better if the gasket sealed off that space.
Also noted is the importance of the transition area, where the coolant moves from the scroll and into the water jacket. It is common to see casting seams and rough surfaces that can be quickly ground out. I go as far as smoothing the outer scroll wall and outer pocket back wall so the coolant flows smoothly into the water jacket.
DD
