do these manifolds work like a bbm or do they remain mechanically the same thruout the rev range?

No, they only have one flowpath.
The Edelbrock performer actually started out with a dual runner length design but they found it too cumbersome and not worth cost of production.
The BXR when available offered plenum covers with moveable walls to achieve the long and short runner control. Mostly this wasn't used because the intake was that good the hassle of actuating it wasn't worth the minimal gains.

Pete.

I put the performer 5.0 on a pretty stock EF XR8 engine (also had genies & cats & B303) the gains were very noticable and the way biggest power gains of all the mods. Good all round gains all the way through the rev range but especially in the 3500rpm+

You definately cant go wrong with the performer 5.0 on a mild engine application.