Boil pasta according to package directions in heavily salted water. Drain and set aside.
In a large skillet, saute bacon until crisp and set aside to drain. Add 3 tbsp of butter to the bacon grease in the skillet. Season chicken with salt, pepper, garlic powder and 1/2 of the ranch dressing mix. Saute chicken in skillet and set aside to drain. Do not drain skillet.
Melt 2 tablespoons of butter in skillet with drippings. Add onions to skillet and saute on med low heat for 3-4 minutes. Add mushrooms and minced garlic and saute and additional few minutes till mushrooms are tender. Remove the vegetables from the skillet and set aside with chicken. Do not drain skillet.
Melt remaining butter in skillet. Add flour, salt and pepper and stir till smooth and bubbly. Stir in chicken broth and heat, stirring frequently, until mixture starts to thicken. Add milk and remaining ranch dressing mix, salt and pepper to taste.
Simmer for approx 5 minutes, or until the mixture is creamy and thickened. Add 1 cup of cheddar cheese and the chopped tomatoes. Stir until cheese is melted.
Add chicken, vegetables and pasta into the sauce. Stir till coated. Just before serving add bacon and remaining cheddar and toss to combine. Serve immediately.