I don't know what the unaccelerated stall speed of the Ghost would have been but 21G would require that the airplane be moving at an indicated airspeed of about 4.5 times that speed. The listed speed of 495 mph seems too high to be an indicated speed. Maybe GS or TAS but not IAS or CAS as that would be close to 550 mph in the turn at over 5,000' density altitude. Average lap speeds didn't even approach that, though top speed down the "valley of speed" could. I'd guess the IAS as being closer to 450 mph max. At 450 mph, the unaccelerated stall speed would have to be 100 mph to make the 21G credible. Interesting. I wonder what that stall speed is...
Still some mystery here.