Mix lime juice, chile powder, oil and salt in a medium bowl. Add steak, turning to coat evenly. Chill and marinade at least 30 minutes and up to 3 hours.
Heat grill to high. Cook steak, covered, turning once, until marks appear and steak is slightly pink in center, about 4 minutes. Transfer to a cutting board, tent with foil and let rest 10 minutes, then slice thinly to fit in rolls.
Open rolls and sprinkle each with about tbsp. cheese. Grill rolls, cut sides up and covered, until crisp and cheese starts to melt, about 1 minute.
Fill rolls with steak, avocado, half the jalepenos and drizzle sour cream. Serve with remaining jalapenos on the side.
YIELD: serves 8