The factory "boost control" can only do so much. Overboost is possible with more flow than the other stock mechanical components can maintain. The stock GT3582R turbo (and the stock FG XR6T turbocharger, although not quite as capable) is far more capable than the other supporting stock parts.
The high flow cat increases flow of air away from the turbo much much faster than stock, whereas removing only a muffler just makes the car a little louder, but hardly increases air flow in comparison.