You’ve pretty much got it right on where you’d want to make your change.
If you’re only using one of the poster layouts, you can just make the change in that section, or you can change each of the “static”, “carousel” and “other” sections if you’re using more than one.
You’d be looking to add extra lines for printing your extra metadata fields, something like (just for example to display Dublin Core Subject):
<?php echo metadata($posterItem, array('Dublin Core', 'Subject')); ?>
You can directly change the file in plugins/Posters/views/public/posters/show.php, or you can copy that file into your theme at themes/your-theme/posters/posters/show.php, and edit it there.