Sidez, out of the nearly 100 cars that I've owned over the years, I've never owned a Turbo, or Supercharged car, so I can't comment on how well headers will improve a "puffed" engine.
I was basing my assumption on what Msheredy said about the Arizona company, "Floored Fabrication" discovering that the oem manifolds actually "shroud" the exhaust ports. I would think that fact alone would cause enough obstruction to hinder good flow characteristics. That's why I think a nice port matched set of headers would probably wake up our "na" engines.
Out of all the cars I've owned, only three had headers on them, and compared to the same cars without headers, they seemed to have a distinct advantage.
There are only 2 sets of factory oem exhaust manifolds that I'm familiar with that even come close to matching what a set of headers can do. Those were the original equipment manifolds that came on the Mopar MAX Wedge engines, and the manifolds AMC used on some of their V8's starting with the 1971 model year. But even as good as they were, headers were still an improvement.