I think it will. It kind of depends how long your tail pipe will be. Pushing the exhausted air from the turbo through a one foot section of 3" vs pushing it through a 4+ foot section of 3" with a couple of bends in it is completely different.
I say go 3.5" to be sure your flow is not being reduced.
Before I had my tailpipe on and after felt allot different as far as spool up goes.
That was when I was only at 5psi. After I did the 2 stage boost controller and cut my cats out is has changed for the better.
Mostly from removing my cats but thats another subject.

 

Good point. Maybe I will just go with the 3" to 4" reducer and then some straight 4" pipe with a turn down. The truck is a daily driver, but only sees maybe 20 miles a week. Im also looking at the Greddy Profec B Spec 2 controller. Seems pretty easy to install and use compared to others Ive seen. Have you seen it? http://cgi.ebay.com/ebaymotors/ws/eB...RK%3AMEWA%3AIT

 

I have not seen it.
I've got the STS cheapy version of a boost controller.
Its works, that is about all I can say about it.