There is no evidence pointing towards faster extraction from either the Refinery or the Extractor. It is not a "gas-equivalent" to a Terran Mule.
The point of this upgrade: (which you point out yourself)
- To spare the 6 or more food supplies to help you get a larger army.
To spare the 6 or more food supplies to help you get a larger army. (or to have SCVs or Drones mine instead - increasing the amount of minerals in the early game)
(or to have SCVs or Drones mine instead - increasing the amount of minerals in the early game) - To take gas with no Command center or Hatchery nearby.
To take gas with no Command center or Hatchery nearby.