The problem is that more often than not, it's very difficult to do right and quickly becomes a disaster. In almost EVERY case, it's a crutch for poor install. When you need upstage tweeters, it probably means you should have spent more time playing with the placement of the tweeters you already have. Now, I understand some people can't (because of leases, etc.) or don't want to tear up a car for complicated fabrication, so things like upstage kits have their place. But like I said, it often turns into a garbled mess.