For the honey-sesame vinaigrette: In a medium mixing bowl, combine the soy sauce, olive oil, honey, sesame oil, rice vinegar, scallions, ginger and black pepper. Mix well to combine. Cover and set aside for 30 minutes.
For the grilled red plums: Preheat a grill or grill pan to medium-high heat. Place the quartered plums in a small mixing bowl and toss with the canola oil. Sprinkle the plums with the sugar. Scrape and oil the grill grates. Place the plums on the grill, flesh side down, and grill until caramelized and slightly charred on both sides, about 10 minutes total. Remove the plums to a plate and set aside.
For the soba noodle salad: Bring one gallon of water to a boil in a stockpot. Add the soba and stir to submerge. Bring the water back to a boil and cook the noodles for 5 to 8 minutes. Drain in a colander and rinse the soba well under cold running water to remove all of the starch. Drain well and refrigerate.
In a large bowl, combine the chilled soba noodles, Napa cabbage, shiitakes, carrots, cucumbers and red peppers. Toss with the honey-sesame vinaigrette. Plate the salad on a large serving platter and garnish with the toasted sesame seeds and grilled plums.