I don't know that you need to consider increasing the speed in a side slip to increase it's effectiveness. As far as my experience goes, you can't drop much faster than a rock, and that's what every aeroplane I've side slipped seems like - a rock. (C150, C152, C172, C172RG, C182, Maule M5-260, PA28-140, PA28-150, PA28-160, PA28-161, PA28-180, PA28-181, PA28-200R, M20C)
And watching some "old dogs" landing short at some back-country strips, it's quite amazing to watch them land almost vertical in a side slip.
I think one should get an instructor that's comfortable with side slips and full flap landings, and try both the side slip and the full flap "dive" method, and see the difference. Both are effective, but I believe that a side slip give you that little bit more control of the decent. (my opinion is mine)