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Greg-
I have posted a bit about custom DPs that I had built through a local fab shop here in NorCal. You are correct, in my opinion and in my observations, in deducing that a split-design (with all other things held constant) should not allow better boost response. The 500RPM (or so) "better boost response" is surely from a) the actual design of the downpipes (curves and angles, sharp bends, etc), and b) the ID of the pipes themselves. Some argue 2.5" vs 3" is no different; although I have my own opinions (based on observations) of the subject, this is not where I am making my argument. The boost response change is most noted certainly because of a) and b) above. The split-flange dps, however, would only add to the stabilization of boost once full boost is achieved. Techinically, there is no reason that a split design would lead to better boost response, albeit a certain advantage with respect to exhaust flow once the wastegate opens. Simple solution: External wastegates (a la Corky Bell) and run separate wastegate exhaust pipes in tandom with the exhaust. More practical solution: Split the downpipe and don't marry the downpipe to the split pipe for at least 7-9" (again, Corky Bell). This is the only reason I didn't personally purchase GD's downpipes. I had heard many, many great things about them, but I wanted to do something different. Frankly, I wanted to go over the top. I thought about the totally separate divorced wastegate, but being a "materials guy", I was concerned about heat fluctuations, and bending moments (and stresses) on the external pipe; being prone to cracking at the divorced part itself. I was also cautioned about the annoying noise it would produce - not what I wanted from my otherwise "stealth" car. So... we designed divorced pipes to meet Corky Bell's "rule-of-thumb" requirement. This by no means is an advertising pitch, and I hope it didn't sound like one. I'm just another member giving my honest opinions based on observations, mixed with a lot of literature review and the physics of flow and turbulence. Good luck, SS
____________________________________________________________________________________________________ “Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius--and a lot of courage--to move in the opposite direction.” 
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