THis is in conjunction with Bob's response.
Nero Burning ROm has a program called CD Drive Speed. Try to have that software, actually it is packaged with Nero's burning rom (later issues), and can be downloaded thru their website (trial version).
In running Nero Burning ROM, the software automatically sets the optimum writing speed so you will not have any recording errors or "slides" in recording. You can change the speed setting though but you have to consider the maximum writing speed of your CDRW.
By the way what is the 48x you are referring to, is it the reading, re-writing or writing speed?
The buffer underrun protection as an added feature for some CDRWs but has no effect in the writing speed. The media you use also limits recording speed, try using multi speed CDR's. In any manner, the CD Drive speed will provide you information as to the speed limitation of your hardware. I had tried using ROxio, Alcohol 120%, Cd Image and Blind Write, but they all provided the same data as Nero's.