Exhaust sizing on turbo cars and NA cars is very different. In theory, you want the largest exhaust possible on a turbo setup. Of course, you can reach a point of diminishing returns at where going larger makes no additional power.

Turbos make power mainly by way of a pressure differential, delta P, where pressure is high before the turbo and low after it. The lower you can make the pressure after the turbo, the more power you will produce and the better the turbo will spool up. Therefore, a larger exhaust will always produce more power and better spoolup of the turbo. The decrease in lag will also translate into more low end torque on a dyno run, as the turbo is creating boost earlier in the power band.

The biggest exhaust you can put on a car would be to theoretically remove it completely and run an open down pipe. I have never heard of anyone complaining from power losses from an open downpipe.

The theory of having too large an exhaust hurting low end torque only applies to NA cars. There is a misnomer that NA cars need a certain amount of backpressure to create low end torque. This is not correct. On an NA car, you want to use the largest possible exhaust that will also create the highest exhaust gas velocity. That means that you want an NA exhaust to flow as much as possible without sacrificing the velocity of the exhaust gases. Of course, when you go larger with the exhaust, the velocity decreases, which hurts bottom end power.

Anyways, the bottom line is that the 4" HKS will be badass on a 240SX. I can only imagine how good it would sound

Peace out,