The numbers are useful and tell a good story, but not the whole story. The tester was not set up to measure the effects of streamlining. . The same axle was used in all Tape tests. The unmodified axle is best, and the double axle is worst. The fastest axle trees are taped on, and the slowest are fiberglassed.
Polyurethane foam is the fastest axle tree material.
Stressing, four-point, and stabilizing bars all hinder performance. The unmodified axle going down the hill is faster than being treed in every respect except one - streamlining. This aspect easily overcompensates all the others. Two axles taped with fir axle trees are slowed by 7.8 inches, but on a hill they help well over a car length in streamlining. A net trade off benefit of plenty.