For the unacquainted, is the purpose of the modification to improve efficiency, and thus get to temperature faster or reduce costs?

Both, sort of. I installed dampers on certain vents to reduce the amount of air that goes to certain vents. This causes my heating and cooling to run a bit longer when it comes on, but it now runs much less frequently and my house has a more even temperature from room to room.

I also did modifications to some ceiling registers because they were never installed correctly and were leaking air into the ceiling of my basement. That helped with the temperatures down there without really affecting anything else in the house.

Increasing comfort can be as big or bigger a driver.