An intake manifold isn't going to give you any power, its going to change where your peak torque location is. In other words, its going to take power from one place and put it somewhere else.

Here's what the ITR IM does on an LS (and its very close to the Skunk2 IM):



As you can see, it guts the midrange and replaces it up top!

If you want to actually learn more about how this works, go to www.team-integra.net , go into the articles section, then performance, and look up the IM article.

 

Yes, increasing the high-RPM breathing capabilities of a normally aspirated motor will take away some power lower in the RPM range. We're talking about a turbocharged motor here though. Since they're peaky already, the gain you see up top is more thn any decrease in the midrange.