If it's a true mixed-surface rallycross event, I would opt for traction control fully defeated. You will to be able to use power to break traction in the loose material to get around corners.
If this is an STI, no issue. Those fancy diffs will distribute power where it needs to go. If it's a WRX, well... it wasn't really built for this. You will probably want the car in "Traction Mode" in order to keep the car from being a one-wheel-wonder. YMMV.
You are a bolder person than I for taking a relatively new car to such a demanding event.