This is all very interesting reading--I'm very much enjoying it while taking a break from doing battle with old/rusty fuel lines. Ugh.
Another odd finding to add to the mix: somewhere on here I came across a super old thread in which (Mark?) Allison relayed the dyno results from his header design for the X. It performed quite well, as I recall. But the really odd part was that he seemed to indicate that a dyno run *with the catalytic converter installed* yielded higher numbers than a dyno run without the cat. That seems counterintuitive to me, but it does seem to fit with the overall sense that you'd really need to play around with various configurations, dyno-testing each, to figure out what works best for a given engine. (And, of course, "best" is contingent.)
My thought is that the single best way to reduce highway-speed engine noise would probably be to remove the back glass window to promote airflow through the cabin. The back glass just seems to act like a big bucket for catching air, allowing the sound waves from the engine to come toward the front instead of being "blown" backwards, away from the cabin. Having ridden a motorcycle with straight pipes, it can be earsplitting at a stoplight but almost "silent" to the rider at highway speeds.