5 inch exhaust too big, pointless??

[MT56POWERSTROKE]

Hello,
I dont mean to ask the same question twice, I just read the other thread on exhaust but I want to know, is a 5 inch exhaust too big? Will it have a positive impact or could it be the opposite if I am only going to be pushing about 400hp when done? I know people might think I am a nut but I have been talking to Reagan at jobeperformance.com and BD has their own program for the edge evolution that is supposed to be pretty sweet. A big mileage increase on extreme, when driven nice, and its supposed to be pretty clean. That is what I have decided on and will be getting one soon. I am going to go on a 2000 mile trip to MN and back so I will have some good testing ground. Reagan got ahold of a edge program a while back that has since been dropped by edge. It was a custom tune somebody made and was a extreme tune and he swears up and down that he gets over 30mpg on a regular basis when driven right and has done way better than that. He has a upgraded intercooler, injectors, turbo, etc too. I have done alot of talking with him and I am sure he is legit and I am willing to trust him. He is trying to get edge to make the program available again but is having trouble, but he says the BD tune is by far the best next to that one. I will let you know what kind of gains I get. I just need to decide on a exhaust before I go on this trip so I can let the tuner work at its best. Please let me know what you think. You can get a 4 inch aluminized afe kit on ebay for 335 shipped, so that isnt too bad. I want stainless but money is tight right now. Thanks