Wingnut, you missed this bit of my statement- "Obviously there are safety margins which make it safe". It's a bit like if you're carrying a Samoan Rugby team v's a bunch of average Indonesians. Vastly different weights. If the company policy is no seat changes then that's the rule.
I think it's safe to say that we know "weight and balance" is just a crap excuse cabin crew are taught to say in ground school and it works 99% of the time.