[spacemanspif]

Sounds like you have already done a ton of homework on the matter and have probably thought it out more than the most header companies. In their defense, they plan to sell to a certain group of car owners who are typically runneing higher RPM than is necessary so the larger tubes are more than likely ideal for the market. Having a Honda you have the wonderful world of used parts at your disposal, I think the easiest way to try this is to buy a used 4-2-1 header and make new primaries. If you can't bend pipe, maybe just weld in smaller tube on the straights for a "quick" test mule piece. Either way I think it could be something worth looking into, many people with small block V8s have claimed increases in MPG after installing headers. Whether or not the Honda manifold chokes the engine as much as the old Chevys did is unknown but the principle is still there.

FWIW I've heard that some of the best custom made headers have Venturis because of their benefits. These are most likely performance benefits but just might help MPG too.