300 is 300 is true, if you use the same pressure drop measurment.
A 500 cfm 2 bbl will only flow 353 cfm, if measured at the 4 bbl flow rating of 1.5 inches pressure drop.
A 500 4 bbl will flow 707 cfm, if measured at the 2 bbl flow rating of 3 inches pressure drop.
That same 600 carb would be 884 cfm at the 2 bbl spec.
Here is a good reference to the comparrasons
http://www.rasoenterprises.com/inde...arb-flow-rating&catid=45:carburetion&Itemid=4