Well, got some less-than-great news about MACH 3.
The audio decoding issues probably can't be properly resolved with the current design of Dexter. This is because Dexter relies on being able to drop both video and audio in order to stay in sync with the NTSC rhythm generated by the Raspberry Pi. Audio output on modern PCs generally is pretty imprecise and thus Dexter needs to be able to adjust the contents of the audio stream dynamically in order to stay in sync. For almost every game, this is something the human player will never notice, but it may mess up MACH 3's audio data decoding. How much will it mess it up? We don't yet know, and I don't think we can afford to hold Dexter rev3 back in order to take the time to find out.
So the current status of MACH 3 is that the "DISC ERROR STAY PUT" messages are not going to be going away. Will the game still be playable? Time will tell.
I am marking PR-8210 support as finished with a note about MACH 3 on the Dexter status page. While Us Vs Them hasn't been tested, I am expecting it to work properly.