You can order sirloin after 4:00 p.m. Monday - Saturday and after 11:00 a.m. on Sunday. While it may not be the best steak cut, that’s easy to overlook when it's all you can eat.
Just make sure to arrive at the restaurant early, particularly on Fridays and Saturdays, as Golden Corral tends to run out of sirloin before the end of the night.