It’s your 4080 doing the work when you’re playing PCVR, the Q3 is just the display.But I am assuming this setting is a matter of the processing power of the quest 3, not the 4080 in the pc, right?
HMDQ is the most immediately noticeable quality difference and sharpness of image is valued by some players above all other things - personally, I prefer all the graphical goodies turned up high/ultra, keep HMDQ & SS at 1.0, then use the Contrast Adaptive Sharpening setting at about 50%. This gives me a similar look to using HMDQ 1.25 but with better performance on my 4070ti.
I’ve never seen a difference in bitrate image quality above 200 so I’ve always kept it at the default with variable rate to OFF. Also worth checking out what resolution is being displayed via the Quest 3 - if you look in the devices tab on the Meta desktop software, you can select the headset and see what is currently set.