You can test https sites with Mercury -- right click a large image from the source you want to test, 'copy image url'... paste it in Mercury and test away.
Some of my favorite Mercury tests that I perform best on are https. So it's not always slow.
e.g. lh4.googleusercontent.com (all of a sudden this one slowed down for me, to like 1/10 of normal - lol - usually a consistent performer), secure.netflix.com, lh3.googleusercontent.com
Since you mentioned this I made it so you can see the URI scheme when you hover. That's useful information.
People in some countries will notice that they're automatically defaulted to servers closer to them than the Dallas server. I still believe that there is more value in a test at distance, these servers are regional. If you're very close to the server, say >10ms, you should definitely test out further to see what you're really getting. (EB I know that you know this, I'm explaining this for others. )