You will get lower spool and 20-30RWHP
Is that from your experience or just repeating something you heard? I'm not sure it would help his current set up. I'm sure it depends of his horse power levels and a few other things. It could be like adding a 3" exhaust to a stock GT Mustang.
As far as spool time and all this. I experienced a little later spool, no other changes. If you think about it, I believe this to be expected. It is like adding a larger AR. Larger AR will spool later and make better peak power not choking off up top. Adding a larger down pipe also reduces backpressure and not choking off up top. A smaller AR will spool quicker and produce more mid range power. It's a trade off, what you lose at one point in the rpm range you gain back in another.
My opinion............on a smaller turbo like that, sub 600rwhp, a 4" dp is not neccessary.