One of my training objectives for the last couple years has been to climb 4000 ft/hr (1219 m) for 15 minutes.
A couple weeks back I hit 1190m (while setting a PR on Kings Mt). So close.
To put it in perspective, I calculate that the "non climber" pros in TdF mountain stages, the guys who are just trying to finish inside the time limit, do the last climb of the stage at 4000 ft/hr for 45 minutes to an hour.