Anyone know if it makes a difference performance-wise if you setup a receiver as a mirrored display rather than an extended display? Seems like a waste/wrong to set it up as extended. However, the one new problem I'm running into lately is that my second monitor (mirrored with the receiver) will come out of sleep saying it's out of range. In which case I then have to change the refresh rate to 120hz, then back to 144hz for it to show up again. It is unfortunate that you can't (as far as I know) just designate it as an audio source and send a strictly audio signal out rather than having to set it up as an additional monitor.
I've always done mine as extended. 144Hz G sync main display 1440P via DisplayPort and Denon AVR 1080p at 30hz via HDMI.
Makes no difference to performance. HDMI is a primarily a video interface and needs to be active for copy protection mechanisms to work. It will never be allowed to work without video.