The only real way to reach a proper conclusion is for an individual to trial it while carrying different weights. There are too many other variables.
The weather alone accounts for up to a minute on my 5k.
My weight hasn't altered in a year but my times have been all over the place.
Yup, I'd like to see a 9st 3lb 2:48 marathon runner run carrying 56lb / 4st ! I'd bet they wouldn't get under 4 hours. It can't work that way round, there's no conditioning to carry the extra weight, it would be ok over a short distance, but 26.2..