The fuel pump provides for more pressure than muffler pressure does.
Generally the pump carbs have a larger bore inside as the pump provides the fuel instead of the engine sucking the fuel. Plus they have a slightly different mid range trim in the carb to account for the extra fuel pressure too.
The fuel tank is rarely inline with the carb as it is usually a little lower. So then the plane goes inverted the fuel flow increases causing the slight rich running condition. The fuel pump tends to aggravate the situation. So yes the slide valve would work for that. But the carbs are setup for pumps, so you probably wouldn't find a non-pump Perry carb with that feature on it.