What I think this is about is, if you put two 1500 cfm carbs, they won’t work to there full capacity as if there was only one. This is a reduction in how much is pulled through the carb. Not the capacity of the carb being reduced or the efficiency. The engine likes the extra available air and fuel flow of the twin carbs over just the single carb. The twin carbs make the engine more power with what’s available in overall A&F flow and lack of restriction.