My " opinion " is that there is power to be gained with a step header..........but you need to build the header to cater to a narrow power band to fully exploit the benefit.
Look at the classes where they are the norm......Comp. and pro stock spring to mind.
Sprint cars and some roundy round guys use them.
Those headers are built with a very specific r.p.m. range in mind.
For a street / strip effort , your gain will be at a cost somewhere.
And for a balls out race car , they are part of a combination of parts that all work together at a given engine speed to deliver maximum performance.
Could you tune them for lower r.p.m. benefits ?
I think yes.
But there is a lot more to it than cutting bends and making sparks , and for anything but a max effort I think they are on the pricey side of the ledger.