Had to check to make sure I had not already posted in this thread. Back in the late 70s-early 80s, I had a 73 Duster 340. Stock except for a spread bore Holley, it ran 15.05 at about 93-94. Put on a cheap set of Black Jack headers, no other changes, ran 14.40s at 98 despite now having a serious traction problem with its open rear (3.21 ratio). Just a sure grip would have got me into the 14.20s. I was amazed at how much faster this almost totally stock low compression motor was with headers.
Fast forward to 8-10 years ago. I sold the early model 340 hi-po manifolds I was running on the 360 in my 67 Barracuda, got rid of the muffler shop 2.25 exhaust and replaced it all with TTI step headers and TTI 2.50 inch X-pipe system. Again, a low compression motor, albeit with thin head gaskets and heads milled.030 (but the pistons are .100 down at TDC). Cam is a Mopar Performance 761, which is basically one step above stock. Again, I was amazed at how much more power it had - everywhere. No traction at all anywhere in low gear; had to swap to drag radials (for the street) because it was scary to drive with street tires (I think roads around Tallahassee are slicker than farther north, maybe heat/humidity). No times, because I haven't been to the strip since, but no question it was a huge improvement.
So then I bit the bullet and put TTI headers/2.5" exhaust in my 65 Barracuda. Now that was a job. Cam is the same as in my 67 Barracuda, compression ratio is higher. Again, big improvement.
So, I simply don't believe any test that says headers are only worth 10 hp. Maybe with a .380 lift cam. ??
Just my own experience. Yours may differ.