In the RAF truckie world we often operate at MAT limits in some of our luxurious? destinations these days.
Have once, a long time ago, been forced by an old bold captain taken off above MAT limit. The result - as we turned at 500' to avoid terrain, the a/c lost altitude, and I lost any respect I ever had for that skipper.
Most weight limits follow a linear extrapolation, and so an excedance has a linear degradation in performance. If you exceed your MAT, it doesn't matter how long your runway is, your a/c cannot physically give you anymore bernoulli's.