2015 exhaust diameter
 

I have heard that we have a two and a half to 3 inch exhaust I'm just trying to find out what it is. Do we have to replace the mid pipe and H pipe.

The cats are 2 1/8th inch i believe... I measured the stock nismo muffler and its inlets were 2 1/4th I think. They are by no means 3, I could be off in the other numbers though

The most restrictive parts of the stock nismo setup are around 2 and 1/8 and are located up front, I had some approximate diameters posted a few years ago. If it really matters I could go and just measure it since it off the car now.

Thanks for the input was hoping to save some money but I guess it's just better to do the entire exhaust then.