Hi Jack!
Stereo coding of an already processed/filtered signal is a zero latency process, whether in software or hardware, so turning it off gains no latency.
It’s possible that the transmitters stereo encoder is as good as BBP’s internal coder. It cannot be better, but it could be worse. I can say this with certainty because perfect stereo coding is an extremely simple process to do in software — you can even do it in an audio editor like Adobe Audition.
Thus, I strongly recommend using BBP’s MPX output if you can.
///Leif