While you will never get the manufacturer's to 'fess up to it, I
wouldn't be surprised to learn that the MGW given in all the aircraft specs,
and the POH, has a significant buffer built into it if only to add some
protection to the manufacturer's liability.
That said, it is still my firm belief that you should never, ever exceed
the W&B envelope. I have heard more than once the saw, "Complacency KILLS!"
So you add 45# over MGW, this week and the next. What the hell, let's make
it 90#, the airplane didn't feel a thing at 45, right? Then, maybe 150 or
even 200.
So you get complacent about the extra weight. And fly out to a patch in
the Mojave Desert like KIYK where the field elevation is 2455 and this
morning the density altitude nudged the 7,000 foot mark. My point is, more
factors exist than just gross weight.
What are you going to tell the investigators if you have a malfunction
totally unrelated to the overweight condition, but they add up the numbers
anyway? The insurance guys will love it.