Great write-up. From a user perspective, grid feeds like Flipboard look great, but I find them a bit less friendly. I'll lose my spot, have a hard time finding the other article I wanted to read, and am generally just a bit more frustrated. I always go back to linear feeds in the end.
This tallies with my personal experience - I stopped using Prismatic after the switch to grid layout, though I never really thought about why. I'll probably give it another try now.
Aside from some statistical reporting quibbles, I think this is a good write up. The authors essentially arrive back to a design that's been leveraged for RSS feeds for ages (e.g., Google Reader, Feedly, etc.) This isn't a bad thing since the design was ultimately data driven.
No shame in doing the experimental legwork to determine that the old way is the best way (in Prismatic's specific case).
Grid for photos as main content, lists for texts...
The lists looked far more visually appealing to me, mostly due to the better usage of images alongside the text. The grids, whilst tidy, felt far less inviting.