The one shown is not an OEM downpipe, but the 2005-> oem ones are open neck. That tells you something, if the manufacturer is now using an open neck design. Before the open neck the oem ones looked like that below.


Both OEM - Left one is 2005 onwards (open neck), Right is pre-2005 semi closed face.

I havent personally tested twin dump ones against semi closed faced and open neck ones.

Twin dump ones were supposed to aid spool up and boost control (of certain turbo's).