Here's the one thing that surprised me the most
The 320x100 block performs just as well as the 300x250 on mobile, tablet and desktop on my (minimally) responsive site.
That's true when I switched a 120x600 to 320x100 as well. This meant moving the ad from the right column to what had been the center column because I eliminated the old right column on all pages. What had been the right column is now the right (main content) column.