I asked a Spit ace about the mirrors (Jerry Collinsworth, 31st FG). He said they were useful on the ground, and useful for formation flying, but not useful for spotting bogies. P-51 and P-47 used the Spit mirrors too, and I assume they had the same value in those aircraft.
P-38 mirror was no bigger, I doubt it was very useful.
The mirrors I've seen in F4U are more extensive, maybe they had some value in combat.