Originally Posted by rallton
replacing the exhaust with anything other than factory exhaust could result in the check engine light coming on an staying on......
I know others have done it, not sure how they over came the computer reading an issue when the new exhaust was added.
Not sure on the answer to your inner or outer question though
I replaced my entire exhaust with headers all the way to the tips and no light.
On the diameter question, I'm sure what size the GXP splits to after the Y but it'* probably smaller than 2" (the pipe before the Y is only 2 1/4" or 2 1/2")