To prepare the steaks: In a small bowl, mix the chili powder, cumin, garlic powder, salt, and pepper to blend. Generously sprinkle 4 teaspoons of the spice mixture all over each steak; reserve any remaining spice mixture for another use. Cover the steaks and refrigerate for at least 2 hours.
Prepare the barbecue for medium-high heat. Drizzle the oil over the steaks and grill until they are brown on both sides and cooked to the desired doneness, about 4 minutes per side for medium-rare. Transfer the steaks to a carving board and let rest for 5 minutes.
Meanwhile, prepare the tacos: In a large bowl, toss the cabbage, carrots, scallions, and cilantro together.
Heat the tortillas on the grill, turning occasionally, for about 1 minute, or until hot. Transfer to a serving bowl and cover to keep warm.
To serve: Cut the steak across the grain into 1/4-inch-thick slices. Transfer the steak slices and any carving juices to a platter. Serve the steak with tortillas, cabbage mixture, avocados, limes, and hot sauce. Let each guest fill two tortillas with some steak and cabbage mixture and top with avocados, lime, and hot sauce.