The open choke plate and shaft presents more of aerodynamic obstacle more than a significant restriction to air flow. Any measured reduction in air flow with the choke parts in place is caused more by the disruption of the flow through the ventures at full throttle. Again, this is the primary reason chokes are not used on race motors.
Using the Holley 600CFM 4160 series mentioned as an example. Theoretically, air flow through the primaries is determined by the area of the throttle bores less the cross sectional area of the throttle shafts and plates in the full open position. Let’s call it about 300CFM for the primaries for argument sake. The choke on the other hand operates within the air horn and does have function to partially restrict air through the air horn when in operation however the air horn has considerably more area and can flow at much higher rates then the throttle bores. Let’s say the air horn total flow is about 400 CFM, then the choke stuff would need to cause 100CFM restriction before interfereing with the flow of air through the bores. The cross sectional area of the choke plate and its shaft in the full open position equates to 30-50 CFM restriction within the air horn? I don’t buy it. Most carbs are rated at final configuration including the choke parts if any, you may be able to pick up a few CFMs by removing the choke stuff but “reduce the cfm of the carb” buy 30-50CFMs?