The ski and boot companies fall into different categories as far as set up and performance, relative to they way the skier's ski on different equipment. Here are the determining factors, and much of it is luck of the draw rather than planned. First, there are bowlegged skiers and more neutral skiers performing on the world cup. Some skiers are knocked kneed, but only a small number. Hansdotter is one that comes to mind. knocked kneed skiers mostly get weeded out before they arrive at the world cup level.
Given those two parameters, I won't go into feet and ankles, because it just opens up too many variables right now, let's look at the equipment parameters first. Boots and Skis from the same company seem to have a bias toward either, over powered, which means too strong on an edge and under powered, too weak an edge hold. To some degree the skier's technique will evolve based on their own anatomy (which can also be over or under powdered like the skis and boots) and the ski and boot propensity toward either performance leaning.
Ideally, and this is a generalization, for sake of simplicity and comprehension, an "over powered" skier, determined by foot, ankle and leg alignment, should choose "underpowered" equipment, so they balance each other out and become manageable. Hirscher, for example would do better suited on Rossignol, than Atomic. By most standards of alignment, Rossignol equipment shows it is under powered or softer. However by contrast, Hirscher has the strength to over power the resistance presented by powerful boots and stiff skis from Atomic, on certain snow, like injected ice. On softer snow, the overpowered Hirscher (his anatomy) and over powered equipment, just don't work as well. In reverse, the same goes for the opposite characteristics, "soft set ups" or equipment. They work better on softer snow.
Are you following this, before I go too much further?