Taking the cushion off the base of the wad will do two things: (1) negate the sealing effect so that gas can more easily blow-by the shot cup, and (2) increase pressure by increasing the inertia of the material that has to be moved at the beginning of the powder gas expansion, when normally the cushion would be getting compressed, now the shot has to move to make space.
I don't know what the net effect of these two competing effects would be on pressure.
If we were talking about guns chambered for .44 Magnum, I would not sweat the issue, because the peak pressure rating for the Magnum is way above the peak pressure rating for the shotgun shell. But, since we are talking about the .45 Colt, with a peak pressure rating not much above the shotgun range, I think your idea needs some pressure-testing laboratory work before trying it at home. Maybe in a large-frame Ruger chambered for .45 Colt, you would have enough margin. But in a vintage Colt, you might get into trouble.
SL1
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