Going from the 40X to a overBurnt 48X is just a trick, like overclocking your CPU, but just making the software push the burner. That and difference between the 40X and 48X is almost unnoticably. So unless you are running a 12X or slower a upgrade will see little results.
That and you need 40X CDR's to burn 40X or faster. That is why I run a 32X burner, I can run all those free CDR's I want.