Thanks for the question.
I am not sure I agree that the Witness is not applicable, though. According to Microsoft's documentation, you can setup a witness in High Performance mode, but it provides no benefit and introduces additional risk. Source (Impact of Witness Section):
I have never personally tried to create a witness in High Performance mode, so I don't know if there are benefits that the documentation neglects to mention. Does anyone more versed care to share some knowledge in this matter?