I just bought one personally and compared it to a stock DP I had laying around. Most Tubular O2 housings have universal bolt holes, my Megan does anyway, which lines up perfectly with the stock DP bolt holes. There's only about 1/4" of extra pipe on the aftermarket flange compared to stock, which is only about 1/8" around the perimiter, so they should seal up pretty well. All aftermarket downpipes have a 2.5" inlet that grows into a 3" pipe a few inches out anyway.
To answer the original question, yes, it will work perfectly.