Any new intake manifold will help you with your output right now man. The reason being is that the more airflow you are pushing out you need the same amount coming in or you won't get the desired power. Like those guys have said before, if you do want this upgrade you will have to start getting new cams, camgears, maybe valvetrain upgrade to feel the great change. Well here are some of the intake manifolds i would recommend, from stock ITR, Skunk 2, Blox and there are many more but this is what i would recommend. I know that you have new headers but you need to do some research in which ones to use and what their outputs are. Here is a helpful site to go to to see statistics.
http://www.importreview.com/performance.html or
http://bob123e.tripod.com/thegreatheadertest/ . There are many mods you can put onto a car but some will give you a little others will give you a lot. You just need to do your research. Well hope that helped you out man. Good luck!