This is a really great case study, but I can't help but think some of the improvement comes from it being the second attempt at the project.

I'd really like to see some case studies going the other way: F# --> C# [or C++].

At the least, you could do C# --> C# [ie complete rewrite]. Any time you do a rewrite with more knowledge of what worked and what didn't, you'll have a better system.

So ultimately, the essay [like most] leaves the question: Is F# better? Or is it just that complete rewrites can provide substantial improvement?