Hi
On a T3/T04 Turbo, the exhaust pipe flange has a diameter of 2.5".
This turbo supports up to 450 HP.

So, can the complete exhaust piping be 2.5" ? Or if it's better to have a 3" exhaust, will it make any difference as long as the turbo exhaust outlet is 2.5" ?

 

2.5 will work but you will want 3 inch for future growth. Save the time and do 3"

 

You can flare a 2.5" to be a 3" without changing exhaust housings. So in essence, 3" is best for 400whp, but 2.5" can work.