I agree that adjustable border thickness would have been nice to have, or just a second font option with thicker black border would be enough.
Secondly, adjustable leading (line spacing)
, OR simply make it X/120% - it's too much now. This really isn't a matter of opinion I'm afraid :p A ratio of 1/1.2 (where 1 is the max height of the characters, and 1.2 is line spacing) is tried, tested, and has been universally accepted probably for a few centuries already.
It doesn't matter if you're making a newspaper, a poster or subtitles for a movie - X/120% is the ratio that gives best readability. Like with everything else there are exceptions - if you're using an nonconforming font, or you're designing and want to create visual effects of sorts. But with subtitles we're not doing any of that, so 1/1.2 is the best choice, I guarantee it
It is not without reason that Adobe InDesign have this ratio as default