The boundry layer (0 velocity air) at the wall of the TB barrels restricts flow. Fewer barrels, less boundry layer. Tip in control for the street using a large one barrel TB is usually done by shading the butterfly plates initial 15deg rotation through a casting/billet protrusion into the bore or by using a shaped plate that flows less air at lower actuation angles...A 2000cfm single barrel TB is going to be a whopping 120mm in diameter...I go 2 barrels for that airflow!