It all depends on the combination. Sounds like you have a great engine/trans/rear axle ratio combo in the 'vette -- and as you mentioned, the carb tuned right. Oh, the way you drive has a lot to do with economy too!
The main advantage to EFI is DRIVEABILITY, not fuel mileage. Theoretically a properly tuned carb engine can get as good or better mileage and power as EFI... most of the time. The main advantage of EFI is that it constantly tunes the engine to the current conditions. Carbs can only compensate so much for weather and altitude changes, and not for quality of fuel at all. So under a relatively narrow range of conditions the carb was tuned for it should do as well as EFI. Change conditions (like seasons or a few hundred feet in altitude) and the EFI engine will adjust to run better while the carb engine would have to be re-tuned to produce the same power and economy. The EFI engine should get better average economy over say a one year period without tuning, and produce better average power over the same period. If you have a fair weather carbed car that only comes out for shows or spring/summer driving you probably won't see much difference between it and a similar EFI engine, unless you're taking long trips with big altitude/weather/fuel quality differences.
Sometimes a bigger engine will get as good or better than a smaller one. Depends on how much weight it's pulling around and how the vehicle is driven. Drive a small motor hard and it will deliver poorer economy than a larger one driven the same way -- in many cases, but not all. It's all a game in getting the "right" combo for the type of driving you're doing and the vehicle you're driving.