I have cutouts on my Cobra. I have a BBK o/r H and they are welded in before the balancing tube.
I pick up a consistent 1 mph trap with the uncapped, so that leads me to believe ~10 gain up top. I have not had a chance to dyno them to see the exact gain or to see if I lose any down low.
Nino97CobraVert had them on his car and he had a dyno show ~8-9 up top with no torque loss down low.