A lot of people don't know that traditional calculations for optimal collector size go out the window when you are talking about 180-degree headers instead of traditional 4-into-1 headers. In order to capitalize on the efficiencies of the 180-degree design, the collector size needs to be substantially smaller than what would normally be found on a conventional header, all other things being equal. That's why proper 180-degree headers have what looks like puny collectors.
Or so I've been told by some genuine engine experts I know. I'm just parroting what they have told me, but they have done countless hours of dyno research to back it up.
At least one Pantera parts vendor produces 180-degree headers with 3 1/2 inch collectors, and they are hopeless, giving up gobs of bottom end and midrange....