Not sure I understand your question about throttling, if you get a plan with 16GB of data then you get 16GB of 4G data that isn't throttled. The 'safety mode' lets you go over your 16GB but at 3G (or worse) speed, so that's what would be throttled. That's not included on every plan, but you can add it (or just buy more data if you need to).
You aren't guaranteed to be able to keep your UDP right now, they can pull the rug any time they want. I have an older plan that was a promo, they won't take it away from unless I change something, same goes for you. Unless, of course, they get cranky and force everyone to change, but I don't see that coming.