I would have to say a 4-2-1 design is the most efficient design due to less negative interaction between pulses that fire close together
Also if you look @ highly efficient turbo setups, they all use divided housings and manifolds. On a inline 4 the pairing would go 1-4, 3-2. And also on a turbocharged car the rule of thumb is go as big as possible on exhaust size to minimize backpressure on the turbine.
Mine is 3"
