Can I see a preview of how links will be rendered?
Sure, you can see an example at the bottom of the settings page for the plugin once installed (although hyperlink colours will be based on your theme and not previewed). For a live preview, see other pages on this site.
Will this slow down page loads?
It shouldn’t. WordPress will initially render the link as a boring hyperlink, which will then get rendered as a onebox via AJAX after the page has loaded. Actually turning the links into boxes happens asynchronously, and it’s highly recommended that you install the APC extension for PHP to reduce the load on your server, as well as the servers you’re linking to.
How can I request extra features for whatever.com
The best way is to open a ticket at GitHub. Bonus points if the site in question has a well-documented API.
What tags can I use when customising the template?
You can use any or all of the following: