Bringing this back up. I just installed 4.6is mufflers and got just a little more sound, still too quiet, though, IMO.
After looking at the manifold to center section transition, I was able to have an educated conversation with several ebay sellers about their fitment. Conclusion is that NONE of the China/Taiwan headers will mate up to stock exhaust on the E53/E39 M54 engine even though several claimed they did. Neither does bimmerbrakes or racing dynamics (to give them credit, neither of those claim to fit but some of their resellers claim E53/E39 bolt-up fitment).
However, after talking with the sellers of "russian ebay headers" aka bmwdoctor.ru, I have some confidence they'll bolt-up to our stock exhaust. They swear it does and sent pictures that at least show the correct flange clocking.
So, with a one-day only 20% off ebay coupon ($450 delivered), I'm giving these a shot:
Russian Ebay Headers
Based on their instagram page, I suspect they're buying them from somewhere in asia and correcting the output side to fit various other models correctly. Over on an E90 forum, someone posted pictures and the comments were they look much better than the standard China-headers but aren't up to par with supersprint.