Running with it cracked can decrease boost by robbing backpressure from the turbo. This can bring up your egt's and with fueling mods, could be enough to damage engine components.
I got the BD, and it seemed to "change" the power. I doubt it added much, but the spool up was noticeably quicker and the power delivery seemed noticeably smoother. My *opinion* is that any multipiece will be fine. It's primary function is to not break, and I'm reasonably certain that the aftermarket manifolds have multipiece design to prevent warping, and are SIGNIFICANTLY thicker. Thicker outside, and smaller exhaust ports inside. The smaller ports increase velocity and decrease volume to assist spoolup.