Well, looks like it was a configuration issue. I just went through the SD card setup process again from scratch and this time it is working perfectly. I must have made a mistake somewhere the first time.
I do have one question for Neo Geo cabs. The SELECT button doesn't seem to be active/assignable. I was hoping to use that to bring up the menu to switch games. Is there any way to make it available? Or is it some sort of non-standard Neo Geo thing...
I have to say the quality of this is a lot better than the Arpicade, the audio amp is much cleaner and I'm getting beautiful sound. I might have to mod the cab and install better speakers now

. Also, there is clearly less lag than on the Pi as I immediately noticed the game felt more responsive and I was able to play better.
Thanks!