Well, from what I remember when I had an accord and looked at turboing, 3" cat back helps a lot, but really wont be quite as noticable with a stock DP and such. I'd reccomend getting 3" down pipe ect. and a 2.5 inch cat back over JUST a 3" cat back. On my 240 I have a 2.5 inch down pipe and 3" exhaust and I should see something like 10whp gain with a 3" dp. Not really sure how much it would matter on an accord or not I am sure the bigger you can make everything the better - up to 3" that is haha.