I had the same question. The only ones I have heard about are the 3.5" downpipes but they are all custom made. Buschur also makes a 3.5" downpipe but it is only compatable with their full system.
If you find out something different let me know also.
Let me see the before and after dyno sheets, video, data logging of boost levels, temperature printouts for both dyno times and any other neccesary documentation to make proof positive of this!!