fuel is fuel. x pounds at y mph requires the same input whether it be carb,s tbi, mfi, di lithium crystals. The advantage to a controlled system, like TBI or better will be cold weather driveability, adaptability to fuel, atmosphere, temp etc.
There is a secondary effect that the absolute cleaner an exhaust can get, the less wasted fuel there is. todays efi systems benefit the jeckyl vs hyde nature of a motor - the epa does not regulate pollutants at wide open throttle. it lets you run a smaller motor at as lean a burn as possible at as low an rpm as possible, yet when the pedal is pushed, pump all sorts of crap down the throat.
If your usage can exist on the limited amount of air a TBI can pass vs the 4bbl, then go for it, you will experience more good than bad. I have seen many people around my shop and others wanting to abandon an efi or even electronic carb system (well who can blame them there) for a carb system because they get scared or dont understand it. On the other hand, for decades I and others have been fitting ford EECIV and lowly GM TBI to about anything. I often (half heartedly) joke that GM TBI is so simple you can adapt it to your lawn tractor. Of course, this works best when you graft the entire motor....carb motors - those from say 72-89 are so detuned MECHANICALLY as well as induction wise, that they can never make power or drive right no mater what you do. If you want to modify the programming of the OBD1 systems - its harder to write phone aps that it is to do that, in fact, there are aps to reflash since the advent of the Palm OS.
The biggest impediment to gas mileage on old squares, is the lack of overdrive.
Still curious about the TBI conversion - I have a history of not liking carburetors. Besides, I'm hoping that for the same price as rebuilding a carb or less I could do the TBI conversion.
