Yeah, I don't know where the "improve fuel economy" myth came from, but that's what it is. A myth. Since the 2 barrel is not a staged one and both barrels open simultaneously, mileage will certainly go down.
IMO, unless you are making several other modifications at the same time, such as headers, maybe milling the head and adding a performance camshaft, the gains you will get just from the 2 barrel carburetor alone and the needed larger exhaust will be noticeable, but not impressive. IMO you will get a bigger decrease in mileage than increase in power.
