I need to know this one too! Just installed the turboxs downpipe but the flange is suited for a flat copper gasket, and the factory midpipe is suited for a donut gasket. Will never get a good seal with that.
I took it down to the local muffler shop, and his suggestion was to cut off the end of the factory midpipe and put on a diamond shaped flat flange to mate with the turboxs downpipe.
Now I'm just considering getting the turboxs midpipe. My only question, is if the end closest to the trunk of the car requires a donut gasket or a flat flange.