I get it that you're a big, strong guy. Understood. I weigh 250ish, and i put in a ton of miles, but i don't stretch chains at that rate.... A lot of big, strong guys ride bikes with # 410 chain; track racers put out a LOT of watts on a high gear ratio on # 410 chain without deforming it at the rate you've described. Maybe you ought to be racing at your local velodrome?
In all seriousness, please consider that even the incredible hulk could jump on your bike and the force of his pedaling would turn the rear wheel and propel the bike forward before it deformed the chain at the highly accelerated rate that you're describing. You could go to # 415 if you'd like, which is stronger, but.... consider the fact that even a 5 year old kid will quickly stretch the chain out on his 16" if the chain is too tight or if the chainline is off.
I see that you got around to measuring the chain stretch; have you actually measured your chainline yet, with a ruler and using centerline as a reference point? A lot of folks stare down the length of their chain and believe it looks straight, b/c that's how a length of chain is gonna look regardless, under tension. But, if you flip th bike over and measure the distance from your front sprocket to the center of the BB shell, and then measure the distance from the rear sprocket to the center of your rear hub, between the locknuts. They need to be within 1mm of each other. Often, even on factory stock set-ups, they are not. If you haven't double-checked with actual measurements, i'd suggest doing so...
http://www.sheldonbrown.com/chainline.html
HTH
