Stock downpipe. When you buy the exhaust by default it comes with a front piece thats necked down with a matching flange for the stock downpipe. According to the product showcase video on YouTube, they offer a 3 inch flange piece if you have a 3 inch DP, or if im remembering correctly if you buy their downpipe it eliminates the flange altogether and just attaches to the system at that clamp. This piece would take 5 minutes to swap out as they use a ridgid sleeve clamp at this part of the system, so you'd just unbolt it and take it out. The pipes butt up against each other inside the clamp so theres no worry of having to pound anything apart.
I don't recall seeing this menu in my car. I'll have to remember to look the next time I'm in it.