This is great data and blows away yet another misconception of mine (like so many have been since coming to this forum). Given the Thunderbolt's reputation for toughness I always assumed it was much more heavily armored than it's contempories.
I believe you, but could you site the source for the figures on the Il-2, its hard to believe a 1940 design propeller plane could get off the ground with 16% of its weight as armor/dead weight.
I can't speak for the precise numbers, but as people said above, the armour wasn't simply protection added to the airframe - it formed the structural monocoque for the forward fuselage (engine/cockpit) with the wings and tail bolted to it. Also, it had a stupidly grunty 1700hp engine.