Every drive can overburn different amounts, starting with 0:00 (not at all), up to an additional 19:99 (on specialized media). Add to that the fact that each disc can be overburned a different amount (some might be able to handle only a few seconds, and some might be able to handle a few minutes! assuming regular and not specialized media), and you've got yourself a combination that can cause some serious frustration if you're looking for regular results. It's a lot like overclocking a CPU, sometimes you can go nuts, and sometimes you can't do anything.
Now with Nero CD/DVD Speed, keep in mind the overburning tests are *ONLY* projected maximums, and sometimes they are 100% wrong (note: this is why I personally NEVER rely on them, and for my reviews, I always BURN a 99min specialized CD-R!).
One thing to consider is that when you overburn a disc, the quality is going to go down considerably with each extra second of audio (or MB of data). So only do it when you need to!
As for what speed to use... it depends on the burner what is best (and again, the media!). Some work best at 12x or 16x, and others work best at 4x or 8x. DO NOT BURN FASTER THEN 16x! It will almost always (I've NEVER seen it not) cause coasters or MUCH higher error rates.
Also, don't worry about the warnings from Nero when overburning, they are there simply so that Nero can cover it's butt if something goes unusually wrong, which I've NEVER seen or heard of happening.
I used to overburn media quite a bit actually (it became a bit of a hobby for me), so feel free to ask if you have any more questions
Punch Cards -> Paper Tape -> Tape Drive -> 8" Floppy Diskette -> 5 1/4" Floppy Diskette -> 3 1/2" "Flippy" Diskette -> CD-R -> DVD±R -> BD-R
The Progression of Computer Media