Okay this is how it is in short.
It's not easy. In case of media there are a few things playing a role.
2 DYE (Is it optimized for high speed ?)
3 Manufacturing quality of media.
Let me go over these and suggest it's influence on speed. (for 16x media)
(1) Can work out good for high and low speed. In some cases Manufacturers got lazy and 4x support isn't as fine tuned as let's say 8x or 16x.
However the other extreme is that 16x was quite crappy implemented and is to fast. So this one can turn out both ways. Most times 8x is the safe option here.
(2) For what is the dye optimized ? Most 16x disc's use disc's optimized for high speeds so burning at low speeds like 4x with good quality will be harder to optimise to not impossible. Still higher speed will be better (arround 8x /12x)if the media is in good shape. (A good dye should handle all types of speed prety well and would only require good support.*)
- The reason why I can burn Mitusishi 16x media good on my Pioneer 106D at 4x !
(3) I allready said good shape. Bad manufactureing quality can cause problems (shape, Bad pressing, Radial problems). Now most of these problems are less problematic at lower speed. So burning at lower speed would be better in this case.
(1) and (3) are reasons why some magazines and some people recommending low burning speeds like 4x even on 16x media.
With really high quality good supported media 16x should be as good as 12x.
In most normal cases 8x-12x should give you the best options.
And incase with crappy media or very poor support for high speeds it might be good to see what happens at 4x.
So I would say start with 8x -12x. IF you really need the time then check how it performs at 16x. ANd if 8x-12x fails you might want to try 4x.
In the really old days 4x/2x (3) (manufacturing quality) was a mayor influence.
in the later 4x/8x area it became much less important.
But with 12x-16x it has come back to importantance specially on the 16x part because of the speed becomming that high that at 16x it behaves differently as at lower speeds.(in some cases the different behaviour can allready occure at 12x)