Don't worry about the fit, it will fit fine. The one piece pipe will fit your car, you just won't be able to remove the cats to be able to put in straight pieces if you ever want to go for the loud setup. I read the market thread and I don't see how the seller wasn't able to clarify if it was the multiple piece pipe, it's not too hard figure out! I would not buy a Bassani one piece catted X pipe only because their cats suck and I would want to be able to replace them without buying a whole new pipe or having to hack it up and re-weld it. But regardless, it will fit on your car.
I would just keep it. It will fit your car just like the "2003" pipe will. The only difference is the multiple piece design. They aren't weak cats, they flow really well. I just had several problems with mine rattling and clogging and needing replacing. I am much happier with my offroad pipe now.