I have found most headers are designed to improve horse power at higher rpm and typically give a reduction in low end torque.
The headers I have experience with did cause a loss of low end torque. I was so happy in one case when the headers cracked after a few years of use. I put the original exhaust manifold back on an got the low end torque back. That was on a 73 Landcruiser and was not a high rpm engine. It worked much better off-road with the stock manifold.
I am not saying headers are not good. You just have to know what they were designed for and make sure the engine will be happy with it.
BTW, RonD is right on with his explanation. Most people don't understand the true mechanisms of headers.
2002 FX4 Bright Red Supercab 4.0 SOHC, manual 5-spd & Tcase, 33" BFG MTs, K&N air filter, Modified air box inlet, Gibson SS cat. back exhaust, Bama flash, Hidden Hitch round receiver, Rear helper air springs, Electric trailer brake controller, Line-X bed liner, WAAG bar with KC lights and Wildernest shell.