I used a 2.5in enthuza and then moved to a 3in enthuza, and I felt like I might have gained 100rpm of spoolup and maybe 5whp. This is on my car on 12psi and pump gas, so maybe when the boost is raised to about 16-20psi I would be feeling more of a difference. I say if you do it once just make a 3" exhaust and then you can sleep at night.