This happened to me 2 weeks ago. A321 c.83tons. Open CLB
Climbing through a long band of Cb around SE Europe at night, passing about fl280 encountered sev turb., a/c stopped climbing and periodically entered descent. Speed decayed to Green Dot -10 in level flight. MCT selected, which unsurprisingly did nothing to improve the situation at this altitude. V/S -300 selected, a/c regained normal speed range in level turbulent flight.On leaving Cb area, Op CLB reselected and back to normal.
MCT or TOGA do not really help at high altitudes, if you enter this situation keeping the a/c under control inside the flight envelope is the key and accepting whatever flightpath that entails, with obvious regard to terrain and other traffic.