Most of the aftermarket headers introduce a flat spot between 5 and 6k rpm. It can be improved with jetting, but if you lean the area too much, you'll end up too lean at higher revs.
Apparently the OEM headers, although rotting through quickly, were a very good shape. I think the expensive Akrapovic system is the best one for not causing the 5-6kpm flat spot.
I've actually filled the majority of the flatspot shown on the graphs by fitting a 4 degree ignition advancer, which seemingly 'helps' the rich area of the rpm range by sparking earlier. Although not really affecting power, the 4 degree ignition advancer is one of the best mods I've made and it only cost me £28!
Engine mods on mine.
Full Micron system
Blanked off Klean air system
float bowl mod
HM Racing springs, needles and jets in carbs.
Made 119bhp at the wheel, but this was before the ignition advancer cleaned up the running in the rich range. Downside is, I have to run super unleaded now.