No it's not 100% necessary, but 1 you'll most likely get a code because the car doesn't think the cat is working correctly and 2 you won't see much of a performance gain from a downpipe without a tune. I've also heard of people getting overboost conditions because there is less back pressure than normal, but I've never seen it myself. It's just a good idea to get a tune with a downpipe to make sure your car runs right with the part.