Okay, the glass effect that you are trying to achieve, have you considered brightening the overall light within the scene to equivalent of the “glass area” and then darken the borders instead of the main area. By reversing the logic, you’ll have about 70% less overdraw (just an eyeball estimate, don’t take it as law). So you would sample the outside of glass area and dim it with transparency rather than brightening the center. The effect is near the same, but less work for the card. Just my two cents worth.
Good luck!