> if you are a really, really skinny person (like myself), you would be looking at a statistically shorter lifespan, unless somehow your normal body temp is lower than average or you spot some other sign of decreased basal metabolic rate. Huh!

I think you are making the error of combining the "folk" understanding of high metabolism - that you are skinny, with the scientific understanding of high metabolism, which means you expend a lot of calories every day. It's more likely that you have calorically restricted yourself throughout your life to become skinny, which has longevity benefits, and it's also likely that very fat people burn a lot of calories every day - high metabolism, worse for longevity.