Place chicken breasts in a slow cooker. Top with 1/4 c. barbecue sauce and cook on high for 4 hours. Remove chicken, drain of excess liquid, and shred. Return to slow cooker, stir in remaining 1/2 c. barbecue sauce, and cook an additional 15-30 minutes.
Preheat oven to 400 degrees. Wash sweet potatoes and prick with a fork. Bake about 45 minutes or until potatoes are tender.
While potatoes are cooking, heat olive oil in a large skillet over medium-low heat. Add spinach and saute until wilted, 2-3 minutes. Season with salt and pepper.
Remove potatoes from oven and slice in half lengthwise. Line a baking sheet with foil and place potatoes on it. Mash potatoes lightly and sprinkle with salt, pepper, and chili powder. Top with wilted spinach and shredded chicken. Shred Monterey Jack directly onto potatoes.
Return to oven and bake an additional 5 minutes, or until cheese has melted. Serve immediately.