I'd love to see some data about that. Even anecdotal experiences - I can't be the only one here who's ported code between languages.
I agree with that LOC/day output figure for new code. I've been averaging somewhere around that (total) speed for the last few months - once you discard testing code and the code I'll end up deleting. Its slower than I usually code, but I'm writing some very deep, new algorithmic code. So I'm not beating myself up about it.
But porting is very different. You don't need to think about how the code works, or really how to structure the data structures or the program. You don't need much big picture reasoning at all, beyond being aware of how the data structures themselves map to the target language. Most of the work is mechanical. "This array is used as a vec, so it'll become Vec. This union maps to an enum. This destructor should actually work like this...". And for something like boringSSL I suspect you could re-expose a rust implementation via a C API and then reuse most of the BoringSSL test suite as-is. Debugging and fuzz testing is also much easier - since you just need to trace along the diff between two implementations until you find a point of divergence. If the library has tests, you can port them across in the same way.
The only data point I have for porting speed is from when I ported chipmunk2d in a month from C to JS. Chipmunk only has a few data structures and it was mostly written by one person. So understanding the code at a high level was reasonably easy. My ported-lines-per-day metric increased dramatically throughout the process as I got used to chipmunk, and as I developed norms around how I wanted to translate different idioms.
I have no idea how that porting speed would translate to a larger project, or with Rust as a target language. As I said, I'd love to hear some stories if people have them. I can't be the only one who's tried this.