[landrovered]

Sure an improper curve can have an effect on the efficiency of the engine but...

The pulse width is the only real tool that determines the richness/weakness of the combustion mixture.

Now, headers could have the kind of effect you are suggesting if the pulse width was already at the edge of the rich or lean curve area and the headers had the effect of pushing the setting over the edge in either direction.

But I cannot imagine that the pulse width would not be set closer to the center of the band to ensure a good mixture and it would likely be set to the rich end to avoid predetonation and micro overheating associated with a too lean a mixture.

It may be the case but I do not think the headers can the cause of the entire problem.