It won't do quite what you have in mind as far as sitting in front of the stock gauges, but I have my monitor on an arm and still use the stock stereo as well as the carpc.
My monitor arm is mounted to the passenger side footwell:
welded to the removable cover for some electrical stuff down there (08 WRX wagon, so yours is likely the same.)
The arm comes up alongside the console and holds the monitor about level with the stock headunit.
I do NOT think my setup will work quite right for you- I'm not doing any autocross, but mention it in case it helps with developing ideas.
The stuff I used was from Panavise.
I have had very few problems with the mounting arm loosening up, and even those could be fixed with some JBWeld if needed, but since it is actually floor-mounted there IS a certain amount of flex/bouncing which would likely be worse on the track.