The guys who are drag racing Titans have a lot of posts on this over at TitanTalk. They seem to be of the view that the currently available headers (all shorty style) do not improve performance. With intake and exhaust mods (in combination) usually Volant and very free-flowing and pretty loud cat-back variations especially in opening up the "Y" pipe diameter, they are getting some improvement in performance. It looks like they are picking up maybe .5 to .7 seconds with the combo and by using all the tricks to reduce weight.
That tells me that our headers are pretty good and we are not likely to benefit from what is currently available. If and when NISMO or some aftermarket supplier comes out with full-length headers, things will change. That type of design will probably really help, but there are serious emissions control issues to be solved there.
Read the posts by a guy called "The Head" over there and then make up your own mind. He has all the good mods and has been running in the 13's with nitrous. He has more experience in this area than anyone I have been able to find.