In a small bowl, combine the chili powder, cumin, salt, onion powder, cayenne, garlic powder and pepper. Sprinkle the spice mixture evenly over the roast. Spread ½ cup barbecue sauce over the top of the spices. Cover and cook on low for 8 hours or until the meat is tender.
Remove the roast from the slow cooker and move to a bowl. Pull apart the beef, removing pieces of fat and discarding them as you come across them. Add ½ cup barbecue sauce to the shredded beef.
Pile beef onto bun bottoms, top with cheese and put bun tops on top. If you'd like the cheese melted, you can warm the sandwiches briefly in the oven or the microwave.