I think the main reason everyone wonders if they can just slap on a 16T is because of reviews like mine that rave about the power increase. The difference is that the stock ecu allows 7psi of boost, which the stock 14T can handle just fine up to redline, and if you slap on a 16T without tuning it you will also be running 7psi of boost, which the 16T can handle just slightly better than the 14T. Now if you run a tune it'll up your peak boost to 14-16psi which the 14T struggles to handle at past 5000rpm, so you feel as if your top end power is weak. Now when you install a 16T, which can handle 20psi of boost, and your tune allows 14-16psi you'll find that it can effciently handle the boost load and easily maintain 14-16psi past 6000rpm, giving you a lot more power in the upper revs.
So just slapping on a bigger turbo without tuning it will show you some gains, but not nearly the gains that a 16T with a tune will show.