MIX barbecue sauce and mustard until well blended. Grill chicken on medium-high heat 3 to 4 min. on each side or until done (165ºF), turning and brushing each breast with 2 Tbsp. of the sauce for the last few minutes of the grilling time. Remove from grill; cover to keep warm.
PREPARE macaroni and cheese as directed on package. Stir in remaining ingredients; pour into full-hotel pan (or into half-hotel pan for trial recipe).
CUT chicken into 1/4-inch-thick slices; arrange over macaroni and cheese.