3" on an NA is a waste of $. Matter of fact, I wouldn't even sell someone a 3" for their NA unless they were going turbo VERY soon. I wouldn't want someone with a poor-performing car to say, "Hey, check out my BRM Exhaust!"
2.4 L is FAR from "big", and unless you're putting 300+ to the wheels, 3' is silly.
Tell whoever told you that to come here so i can schmack 'em.
The piping sizes weren't tested on the same car, that would be senseless. The 2.25 and 2.5 were tested on a stock KA with minor mods, and the 3" was tested on a 330 rwhp SR20DET.
p.s. Gaining "a lot more top end" on a car that has a speed limiter does you no good either. Besides, who does 118+ in a 240?