So presumably that eats into the 512 item limit. If I had quite a deep directory structure (10 levels doesn't sound that unreasonable) I'd only get this speedup for commits changing 51 files or less. Right?
In your calculation, you are assuming that all of those files are in independent parts of the folder hierarchy. In practice, deep directory structures mean that single commits are likely focused in a few very deep folders. We ran the statistics on a number of repos with different shapes to come up with this number, and it covers 99% of commits that we measured.