I forgot to include this link to explain how to actually change those settings:
HOW-TO:Modify the video cache (wiki).
There is also some "tool"/helper add-ons out there that will set some of these cache settings automatically. I'm not all that fond of those add-ons, but that's only because they don't explain what the settings do, so sometimes people choose the wrong one. For example, the "zero cache" mode should be avoided for devices with a small amount of internal memory and only used if you have at least 16 GB or more. (8GB devices typically only have about 4GB of free space after the OS and Kodi are installed. So as long as you know what the settings do, then it should all be good.
Like I said, this really just smooths over the number of times Kodi will stop to buffer. If it needs to buffer then it will always buffer, but having to stop and buffer less often is still better than nothing. In some cases you might not even see any buffering because these settings allow Kodi to fill the buffer faster when there are squirts of faster speeds, which is why they're very useful for people who have fluctuating wifi.
As for your v15 question, that sounds like a good plan. Sometimes the pre-installed versions, regardless of the bootleg add-ons, come with extras installed that you just don't need and might possibly bog things down (if they run in the background, for example, or just making updating add-ons faster, etc). Installing just what you want is a good way of making the box a little leaner, and reduces the number of things that might go wrong ;)