My opinion is that while your carb may not be the optimum size, it is the tune in the carb (or maybe other issues), not the size of the carb that is your mileage problem. The carb being a bit oversized could contribute to the bog when the secondaries come in, but that could be eliminated with a proper tune as well.

With all the problems that you are experiencing, I am not convinced that your buddy does know what he is doing when it comes to carbs.