Make the pimiento cheese: In a bowl, mix both cheddars, the cream cheese, pimientos, mayonnaise, chives, garlic powder, cayenne and paprika. Season with salt and black pepper. Add the reserved pimiento liquid to loosen slightly. Cover and refrigerate until ready to serve.
Make the patties: In a bowl, combine the beef, mushrooms, Worcestershire sauce, onion powder, paprika, garlic powder, 1 1/2 teaspoons salt and 1 tablespoon black pepper. Form into 6 extra-large patties, about 8 ounces each. Make a little indentation in the center of each patty.
Preheat a grill pan to medium-high heat. Grill the patties for 4 minutes on each side, or until the internal temperature reaches 150˚ F to 155˚ F (I like my burger medium well), topping each patty with a large dollop of the pimiento cheese during the last 1 to 2 minutes of cooking. If you like your patties more on the rare side, take them off the grill when the internal temperature reaches 140˚ F. Remove the patties and cover with foil for about 5 minutes. While the burgers rest, slather the buns with melted butter and place them on the grill for about a minute (keep an eye on them to avoid burning).
Make the sauce: Mix the mayonnaise and barbecue sauce in a small bowl. To assemble the burgers, spread the barbecue mayonnaise on the bottom buns. Top with the lettuce and tomatoes. Add the burgers, red onions and pickles. Place the bun tops on the burgers.