Preheat a grill to medium high. Combine the five-spice powder, brown sugar and cayenne in a small bowl. Butterfly the pork: Slice lengthwise down the center, stopping about ½ inch before cutting all the way through. Open the pork like a book and flatten slightly with your hands. Lay a piece of plastic wrap on top and pound with the flat side of a meat mallet or a skillet until ½ inch thick. Season with salt and black pepper, then rub all over with the five-spice mixture; set aside. (This can be done up to 2 hours ahead; cover and refrigerate.)
Make the dressing: Whisk the vinegar, honey, soy sauce, mustard and ginger in a small bowl. Slowly whisk in 1/4 cup vegetable oil until smooth. Refrigerate until ready to serve.
Brush the pork with 1 tablespoon vegetable oil. Grill until well marked and just cooked through, about 6 minutes per side. Transfer to a cutting board and let rest at least 5 minutes.
Toss the plums and scallions with the remaining 1 tablespoon vegetable oil and season with salt and black pepper. Grill, turning, until lightly charred, about 3 minutes. Transfer to the cutting board.
Combine the cabbage, snow peas, carrots, bean sprouts, mint and cashews in a serving bowl. Chop the pork, plums and scallions and add to the bowl along with the dressing; season with salt and black pepper and toss.