There was an interesting comparison test of headers in a recent issue of HotRod Magazine comparing engines running open headers, then into full exhaust, with and without collectors, with and without collector extensions. I don't specifically recall this article dealing with equal length vs non but that would be something I'd look for at the magazine's website. Here's a link that I saved that also mentions header issues:
http://www.vetteguru.com/mods/headers/