1. Heat gas grill to medium; heat charcoal grill so that most of the coals are stacked to one side. Mix together 1 tablespoon of the taco seasoning (save remaining for potato salad) and the chili powder. Rub mixture into steaks.
2. Place steaks on grill (for charcoal grill, place steaks in the center of the grill where the heat is moderate). Cover and cook for 5 minutes. Turn over and grill an additional 5 minutes or until steaks register 145 degrees F to 150 degrees F on an instant-read thermometer. Remove to a platter and cover with foil. Let rest at least 5 minutes before slicing and serving with the potato salad.
1. Boil potatoes 10 minutes or until tender. Drain and rinse; cool completely. Stir in red pepper and scallions. In small bowl, whisk sour cream, mayo, and taco seasoning. Gently stir into potato mixture and top with shredded cheddar, if desired. Refrigerate until serving.