The recommendation of 1x is not for the drive, but for the Karaoke Player you will use the written CDR disc in.
Some Karaoke Players won't play a CDR disc written at faster than 1x. The notorious one is Sony, and those that also play DVD discs are also likely candidates for requiring 1x.
Depending on your computer processor and hard drive speeds, and if you have DMA operation enabled for the CD-Recorder (Plextor Drives especially), you should be able to write with the Plextor 12x at...
12x, or the 16x at
16x.
Therefore, the decision of what speed to write at is more dictated by the Karaoke Player(s) you will use the discs on. Make a few at higher speeds and try them out on the Player. Be sure you make notes what speed each was written at so you know. Then you have some real-world data to decide with.