No, cut shells do not turn birdshot into "basically a slug". They really don't change terminal performance of shot in any substantive way. They are a field expedient way to keep a very tight pattern at significant distance. As such, they could increase the effective range of buckshot. But they can break apart in a pump action, leaving tiny birdshot pellets jammed all over inside the action, taking the smoke pole completely out of a fight. They also have the potential to leave a bore obstruction behind and blow up your gun if you don't check carefully after each shot.