The holes are there to balance the suction forces on the impeller. If they weren't there, the impeller would have a constant pulling force against it toward the suction.
As far as moving the impeller rather than flattening the plate? That doesn't do anything to reduce the clearance volume of the pump. Within limits, the reduced clearance volume reduces slip and increases efficiency and velocity.
Looking at the two impellers in the pictures, the old pump would be the most efficient because there is much less clearance at the tip of the impeller. Larger clearance allows more fluid to be recirculated, back through the holes on the backside, to the suction. This reduces the amount pumped. The fluid will take the path of least resistance back to the suction. You can see how moving the impeller on the shaft will increase the backside clearance. Flattening the plate will leave that the same while increasing the pump's output.
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Rich
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