Dispite the larger primary bore size, mileage shouldn't be effected IF you can tune the carb back to the same state as the smaller primary carb. Throttle response will dip a little bit between the 2 carbs.
The carb I have with its stock jets and rods match with a earler model for a 340, the only difference being the bore size. The throttle response I have is a little flat with the bigger bore.