Nick,

I agree that exposing more surface of the duct to attic air will waste
some energy. But if you read the original post, he is having trouble
with ducts sweating, not a high electric bill. So I addressed HIS
problem.

You show a supply air temperature of 40 degrees. I have been measuring
supply duct temperatures for 30 years, and I have never seen one that
low. Normally 50 to 60 degrees, depending on air flow and return air
temperature. Most often around 55 degrees.

If you ran a supply air temperature that low, you would need special
controls to prevent evaporator coil freezing.
That low of a supply air temperature would definately increase heat
gain through the ducts and waste energy. Also, the capacity and
efficiency of the AC would go down.

We once modified a 25 ton chiller with a factory installed cold water
kit to run at 25 degrees chilled water temperature instead of 42
degrees chilled water temperature. We added about 50% glycol to the
chilled water. The capacity dropped from 25 tons to 10 tons. The
efficiency dropped also.

Stretch