are shumacker headers restrictive?

I will tell you 1st. hand that they are restrictive once you get close too 500 FWHP, I had them on my 451 low deck, car ran decent, then i installed some FW 1.7/8 Hookers & it was a whole diffent animal through the "intire" powerband, car went .5 & 5 MPH faster, so YES they can/will be restrictive, they are a small stepped header, there not designed for MAX performance, they were never intended for this, but there better "somewhat" then high flow manifolds but not by much, much better then smog mannys, not even close too FWs or TTIs, now as i say that, i will say if you have a "mild" BB & tend to keep it that you can't go wrong with these headers.