sdennison wrote:
How thick was that hardened steel plate? Pretty standard for most single seat fighters? How successful was it or do we know?

Read Bob Johnson's book about being a P-47 pilot. He was unable to maneuver much, unable to bail out and hid behind the armor plate while the Germans shot up his P-47 but survived and flew home. His P-47 was shot so many times they quit counting holes IIRC.
P-51D header tank and firewall protection are 1/4" thick. The pilot seat piece is 5/16" from bottom up to the head part that you see in the canopy. That is 7/16" thick. About where the head part angles forward there is a splice plate welded on to join the 5/16 lower part to the 7/16 top part.
The armor plate is surface hardened on 1 side only and is marked as to which side is soft or hard so you face the hardened side toward the shooters.
Rich