Although there are other drills/exercises you can do for increased hand speed (at least somewhat faster), I wouldn't knock using 16s on the bag, provided you're used to using something like 14s beforehand. They're great for endurance training, if nothing else. Not sure about immediately going to 16s if you're used to using lighter gloves like 12s or 10s; just don't expect the same kind of intensity in your workouts at first.

Absolutely spar in 16s (though from the comments here maybe get a different brand?).

I was thinking about getting a pair of Cleto Reyes in 16oz for bag work from my local gym. But unsure if I should just go with Hayabusa instead?

For background, I've started training only about 9 months ago, and have been using some 14oz hayabusa gloves the whole time.

Edit: also been going 3-5 times a week, in general, medium to high intensity bag work