The Perrin 3" downpipe has a 3" flange to attach to the exhaust. You can flip over the stock "donut" gasket and mash it into the 3" flange to mate it to the stock (2.5") exhaust. Not exactly "race-grade" but it works and doesn't leak. Build quality and welds are outstanding on this pipe, plus the divorced flange on the turbo side offer a more stock-like configuration than some "bell mouthed" downpipes do. Subaru has the wastegate seperated for a reason in my humble opinion.
Im hoping this well help defend against boost creep when i throw on the 3" turbo back.
There are also 3"-2.5" adapters that would provide a less turbulent connection, but your tailpipes may stick out a little more with this in the mix.
I plan to go full 3" turbo back in the future and i would rather have the jerry-rigged setup for now, knowing that the 3" flange will mate up perfectly with an aftermarket 3" whenever i finally get around to picking one out.