The good folks over at TSST wisely limited CMC MAG M01 to 18x. Most of the time, you don’t want to write that fast with CMC MAG M01, anyway, and should try 12x most of the time. It’s just not that good of a media to start with.
For most other 16x media, this will also be the case, simply because they are just not good enough for 20x writing.
If you want 20x, most Verbatim 16x media will do well at 20x, and is allowed to burn that fast. 16x media made by Taiyo Yuden is the same way. I still wouldn’t recommend doing 20x on every burn [for potential write quality issues], but it’s workable.