Rated 4 out of 5 stars

Given Maxime RF's very prompt reply and the promising note that it bears, I'll say Thank You to Maxime and will increase my rating to 4.

Regarding what I previously said about the additional features of this add-on, they'll remain of no interest to me, but if this add-on's present problem with continuing to buffer videos when I've selected the option for stopping autoplay gets corrected in the next version of this add-on, then I'll have the add-on still installed and it'll be updated. At that time the add-on will be re-enabled to give it the "stop autoplay" test again.

On a last note, why not change the options. F.e., there could be two options and the second would possibly be activated only if the first one is selected. The first would be for a user to select whether he/she wants to prevent videos from autoplaying. If the person sets the option for stopping autoplay, then the additional option would be to have the user indicate whether or not videos should be buffered. This would permit users to use both of these options according to whatever combination a person prefers. F.e., I'd choose to stop autoplay as well as for there to be NO buffering.

That would leave far more complete control in the hands of each user; for some, perhaps many, users would prefer no autoplaying as well as no buffering when a video isn't playing. This way, people with slow and others with sufficiently fast connections would both be able to benefit.

Oh, and why can't HD videos be prevented from buffering when the videos aren't being played?

This review is for a previous version of the add-on (1.5.1-signed).  This user has a previous review of this add-on.

Hey Michel,
I will try to implement a "Stop buffering" option like you have suggested, but for now there is no way to use the YouTube API to stop buffering... The first idea that comes to me is to unload the video and replace it with one of its thumbnails, then load the video when the thumbnail is clicked... Maybe it will do the trick.
If you have questions or suggestions you can contact me via email.