Yeah, the downpipe is 2.5" at the precats, then needs to converge to a 3" pipe. I wouldn't want a true dual system though... even with the smaller 2.5" pipe all the way back, it would be much heavier than stock. See about getting at least a downpipe like the stillen downpipe (eliminates the front precat and main cat) while the rear cat stays. However, some speed shops do sell precat eliminators that are 2.5" pipe with a flange for the sensor. AHHGGH! The problem I have with exhaust is that there are simply so many different kinds to get, and not all of them are compatible with each other. Some downpipes eliminate the front precat, some do not, some downpipes eliminate the main cat, and some do not... basically the only way to buy an exhaust with the least amount of headache is to make sure that you get it all at once and you know it all fits together easily. To that end... why not have a vote to build a Stillen type downpipe (which by most accounts is the best you can get for our cars, price nonwithstanding) and if possible, have it eliminate the rear cat as well, a 3" test pipe built to fit to the downpipe, and a 3" cat back system, that can either go to one huge muffler, or dual mufflers like the stock set up. Personally, I just want the exhaust on one side to save some weight, since the extra pipe coming out the back doesn't help performance any really.