IANAG have an interesting article, albeit brief, discussing the difference in burn speed times with the test subjects being 40x12x48 and 48x12x48 burners from Lite-On. I found this information especially interesting since I'm currently on the market for a new burner, and you should as well if you're in the same boat.
I've been asked a few times recently if it's worth upgrading a 40x CD burner to a 48x burner. In my opinion, I didn't believe that 8x more burning speed would make a huge difference, and guessed it may take 10 or 12 seconds off of an already quick 3 minute burn time. Well, today I ran down to my local computer shop and picked up a Lite-On 48x12x48 CD-RW. For the past few months I've been using a Lite-On 40x12x48 and it's probably the most reliable piece of hardware I own. Due to that, I decided to stick with Lite-On when I was choosing a new CD-RW.More information @ IANAG