I'm developing my own text editor. To test naive implementation of text editing routines - array of line strings. I concatenated all sources of recent Linux kernel 4.6.3 once. From what I remember it was all .c, .h and .S files. Resulting file had 542MB and 19'726'498 lines. It was good enough even adding a line at front.
[The actual analysis is much more subtle, as the tree invariants are a minimum and maximum number of children for non-root non-leaf nodes, and a minimum and maximum leaf size. When freshly loaded, the code is careful to maximally pack both, and after extensive editing the distribution will be more varied. However, it's still rigorously O(log n), so it ends up being a small constant factor. Might be worth doing some empirical performance testing.]
At the moment I am also writing a text-editor with my partner, to show-case the C++ RRB-Trees implementations in Immer [1]. We are just started, but my plan is to stop at 1000k lines. Interestingly, with such persistent data structure, you go already a long way in implementing undo, parallel processing, and many editing algorithms are super simple thanks to slicing/concat. Memory consumption is still sub-optimal (also I am using wchar_t to simplify my life), but I am very satisfied with the results so far (it is the fastest editor I have on my machine when editing 1GB file, also can edit a ~100MB file on a Raspberry --- larger files fail only due to excessive memory use :/).
[1] https://github.com/arximboldi/immer