probably not a noticeable difference. put the money towards a better cpu/gpu if you have to spend it.

No its not the fastest, there is a 2600mhz kit from the same manufacturer, pretty sure I've seen faster too.

If its mostly for gaming and browsing, then 16gb and 1600mhz is fine. Faster doesn't hurt if its not much more expensive.

If you're doing graphics editing with large files, you might prefer 32GB. But when I say big I mean BIG, like 4GB etc.

Some programs can really benefit from a fast RAM speed, so it depends what you want to use it for. Best thing to do is to look at some reviews and compare it to 1600mhz with the programs you use. Most times it won't be necessary though/

2133MHz is the maximum speed supported by that particular motherboard.

I'm not sure if you exceed that even with overclocking.

Super-fast memory is used to stabilize higher CPU overclocks. For games, there really isn't any measurable increase in performance beyond 1600MHz cl9.