I didn't read the whole blog series, but this data structure seems like it could support O(log(n)) insert/remove in middle or front. The equivalent operation in an ArrayList would take O(n).

Why not show off this performance advantage in the benchmark?

Clojure's built in PersistentVector does not support O(log(n)) inserts/removes from the middle/front, but there is another implementation that shares a lot of it's implementation with some extensions that does: https://github.com/clojure/core.rrb-vector/

Nit: most common cache line size is probably 32 bytes, due to smaller processors outnumbering desktop chips.