[tguvot]

If you are talking R-values - you are right. But if we get a bit more practical, walls have a bunch of thermal mass (drywall on the inside, stucco on the outside, at least in my case ). During the summer, i have walls that get to ~30C on the inside. So even when it's evening/night, and temperature outside drops to 15C from 35C during the da, I open windows and run whole house fan, those walls keep on heating the air inside while lowly R rated glasses shed the temperature quickly. So in my case doubling insulation in wall (in order to prevent drywall from heating up) likely to provide more effect than bumping R value of the glass.