Unless my alcohol fuddled memory is playing tricks, I remember tanking a Lightning off a Victor at FL430 because I was fed up of bouncing around in cloud. Had to use reheat to stay in but never made it on top of the cloud.
Took another Lightning to VFR on top, but was definitely not in stable flight. Above FL500 was easily attainable (subject to fuel), but was also above the pressurization/oxygen regulator release to service.