[projectRDM]

You guys can't be serious. This is one of the most common, well known things about a car, period.

Have someone sit in your car and press the brake pedal firmly and watch the booster/master cylinder. The firewall will flex a little from the pressure. You can visually see it move a little forward, then retract when the pedal is released. Same as if you pressed against a quarter panel with your foot, the pedal is only mounted to hold it in place, the sheetmetal used in the bracket and the firewall are both thin enough to allow for plenty of unwanted flex.
The master cylinder stopper bolts the shock tower (where the steel is a little thicker) and has an adjustable pushrod to press against the cylinder, keeping it from moving. It works, several of the guys I autocross with have built makeshift versions. The only issue with the Cusco is it's for a RHD car, there aren't the same mounting provisions on a LHD car's shock tower for it. It's adaptable, but for the money you can make one that works just as well.