I would say this depends on your progression.
While you're learning, the sidecut shape does not matter as much because you're not going to be carving a whole lot of your riding...at least not to the point where the "sidecut profile" really matters. If your board is directional, it likely has a significantly longer nose than tail. Therefore, when you are performing SKIDDED turns, riding switch will be harder because there would be more rear edge to drag on the snow...making it harder to turn. It's the basic principle of leverage, where you would put your hands more toward the end of a tire iron to producing more force on the nut. So it would take more work to skid the board. You can definately feel it, especially when you're much better, which is why they make twin boards.
I don't know, I'm no instructor, maybe it is easier to center your stance on a directional twin, I just don't understand how that is going to make riding switch easier when a directional setback twin is already designed to ride switch, its just performance orientated for riding regular.
Extreme placement to center the stance is simply going to move your axis away from the waist of the board and put you on or in front of the flex point of the tail.
To me that sounds like it is going to make the ride worse both in switch & regular.
Now I understand your concern about performance, but skidded turns? That technique is not far away once the rider has learned to get on edge. And while it's true you'll get equal performance from the board if it's a true twin, the rider is still going to have to learn to ride the board switch efficiently before they can expect any performance from it.
Maybe one of the instructors can share the knowledge.