I am currently looking for a new exhaust system for an MY01 WRX, I see Scoobymania and a few others tend to offer 3" bore for the Pre MY01 model however magnex have now released a cat back system with a 2.5" bore for the MY01. What are the difference between the bore options besides the obvious

What is the bore size on the std exhaust and considering future modifications can a 3" bore downpipe (Scoobymania) mate to a 2.5" cat back system.

cheers

Paul