The reason for the 48-52x limit is the rotation speed, 9000+ RPM. Faster than 9000 RPM rotation runs the risk of cracking and breaking the polycarbonate that the CDs and DVDs are made of.
CD-RW are read at slightly slower RPMs because they have lower reflectivity and are likely to slow down when read at 48x at the outer edge.
Finally, some manufacturers are a little more conservative than others. The difference between 40x P-CAV and 48x CAV reading is probably within 20 seconds, no big deal, especially in a DVD burner.