You should be aware that the performance increase is virtually negligible between a 40x and something around a 24x. To quote plextor's site, the 40x is capable of "40X speed, the new drive is capable of burning a CD in just over three minutes." This doesn't make any sense as my 12x burner is able to burn discs in approximately 7 minutes, but the difference between 3 minutes and 7 minutes is basically 60% or so. The real fact is 40x is a MARKETING gimmick, the MAXIMUM speed at which the burner can write, most often in the lead in/out areas of the disc. When actual data is being written to the disc, you are running at a slower speed...
Edit: Someone else probably knows more about this than I do.