So as I understand it the standard carb is over flowing right now?
Sounds as if your needle is not seating fully if this is the case.
Remove the bowl and push the float up by hand while it is installed on the engine and turn on the petcock. See if you can shut it off yourself.
The other carb could definetly cause lower rpm issues. If the jets and needle swap over from the standard carb i would do that as a starting point. A bigger carb may mean less venturi (sucking of the fuel from the bowl) and this would be the disavantage to lower end. At higher rpms size starts to not matter with venturi. Size of the carb can be a limiting factor to how the engine breathes at higher rpms. Other limiting factors are port size, valve size, exhaust size, and air filter size. In other words there are a lot of factors involved here. Experiment but try not to change too many factors at once. Start with Carb with old jets then put one jet in at a time to see what works for you.