OK, you're right that there are two different issues here.
The accuracy of the instrumentation after going to larger injectors. Whether or not actual fuel mileage is affected after going to larger injectors. We know the answer to #1. For #2, my gut tells me that mileage will drop. But, as someone already said, it probably depends on a combination of factors including other mods and how you drive. In practical terms, though, anyone who goes to the trouble of modding the car is going to drive in such a way that mileage will suffer.