Ok that makes sense. Unfortunately I think it's impossible to use Repetier Host in that way because in not-ping-pong mode it still waits for oks to send commands when it thinks the buffer has filled up. I'll check if I can another host working though. I have noticed that debug mode tends to come on the whole time and possibly destroys the serial bandwidth? I have no idea what is causing it but will keep searching.
The Duet does send the OK response so it does work if you disable ping pong.
Repetier seems to turn debug mode on by default but you can turn is off again, you just need to press a couple of buttons in the UI.
PanelDue doesn't send high volumes of data, so it doesn't need flow control.