Place balsamic vinegar, oil, rosemary, salt, pepper, garlic and pork in a large resealable plastic bag. Seal and refrigerate for 12 to 24 hours to marinate. To prepare coulis, puree plums in a blender or food processor. Press mixture through a fine mesh sieve to remove all skins and solids. Place plum puree in a small saucepan with wine and sugar; simmer over low heat for 10 minutes or until slightly thickened, then stir in rosemary. Let cool. Grill pork over medium heat until it reaches an internal temperature of 145°F. Let rest for 5 minutes then cut into 1/4-inch thick diagonal slices. Pour plum coulis on a platter and top with sliced pork. Garnish with additional fresh rosemary and grilled plum slices, if desired.