There is no evidence poiting towards faster extraction from either the Refinary nor the Extractor. It is not a "gas-equivelant" to a Terran Mule.
The point of this upgrade: (which you point out yourself)
- To spare the 6 or more food suplys to help you get a larger army. (or to have SCVs or Drones mine instead - increasing the amounts of minerals in the early game)
- To take gas with no Commandcenter or Hatchery nerby.