In small bowl, mix butter and 1/4 teaspoon of the lime peel. Set aside.
In large shallow dish or resealable food-storage plastic bag, mix margarita mix, oil, salt and remaining lime peel. Add steaks; turn to coat. Cover dish or seal bag; refrigerate at least 30 minutes to marinate.
Heat gas or charcoal grill. Remove steaks from marinade; reserve marinade. Place steaks on grill. Cover grill; cook over medium heat 10 to 12 minutes, turning once and brushing frequently with marinade, until steak is desired doneness. Discard any remaining marinade. Serve butter mixture with steaks.