well, a "snug" fit in a pump is somewhat relative. If your talking high pressure, then yes, but most all of the pumps that we use in the hobby are centrifugal pumps, and while the clearance between the impellor and the vanes, and or houseing is important, its not so important for us...
now, let me clarify that a bit, if you take a substantial amount off the actuall impellor, then yes it could effect the performance of the pump, but it wouldnt make it work harder, if anything it would lighten the load on the pump, cause it wouldnt be able to "grip" the water as well to try and push it out the output... so it "slips" and less water is moved, but the pump still turns at the same speed, and it still draws about the same power from the wall...
in this particular situation, the little plastic impellor dosent weigh hardly anything compared to the magnetic rotor. The one thats encased in the stainelss tube that i "rust tested" before trying to use this pump. My thinking is that if i can do most of my ballencing on the magnetic end, and leave the actuall plastic impellor as is stock, then it should effectively make the pump a bit more efficient, cuase it will move more water, cause the impellor will be staying more centered in the housing of the pump, rather than wollowing around with a variable and changing gap all the way around...
Your thought process is good, just not exactly on point for this kind of pump..