I imagine you'll find smaller to be better. I'm having a brand new header made for my race Z and it has itty bitty primaries to keep the velocity high. Look under any up-to-date race cars and you'll find much smaller primary pipe than you did ten years ago, with more attention to how the collectors merge and the various steps inbetween.
Burns Stainless - Home
Is a great resource. Fill our their engine sheet and they'll do a design program on it and give you the details. They are not cheap, but they are highly regarded in race circles.
Ron