Hi,
last update v 1.5.8 is activated rearview mirror for Spitfire. It is now thanks to this possible to use the mirror in Normandy/Nevada, if you don't want to wait for an update for DCS 2.1 you can use my JSGME mod.
file: https://www.dropbox.com/s/7arz9h9z8ubbs60/Spitfire%20Normandy%20rearview%20mirror%20on.zip?dl=0
Edit:
I performed several tests :) under 2.1 and activating the frame rate does not limit or very minimally.