There may also be significant differences between properties measured in compression,tension, and shear.
Many composites are anisotropic.
2 does seem to be a reasonable correction factor for many materials. When you want to compare the mechanical properties of samples of essentially the same material, it may be sufficient to perform the actual mechanical testing on one of the samples to determine the correlation between this data and ultrasonic velocity. Some composites exhibit different ultrasonic velocities at different frequencies as well as in different directions. It is much easier and more reliable to compare the impact of some processing variable on the properties of one material than to try to generalize about a group of materials. There may also be significant differences between properties measured in compression,tension, and shear.