Will any releases happen before the v5 upgrade is done?
It is likely that the release will happen in a few weeks, and since we _really_ don't want to have to release a fixup due to the difficulty in getting the release team together, I suggest a change like Scintilla v5 probably won't happen before due to the size of the change.
Having looked at your change, its not just about speed, but correctness.
As it is now the compare compares the two pointers, not the string contents. I'm not sure what Unique_string is used for, but its always going to fail to find the string and make a new one, so the list will get long and the compares get slow since it will do a linear compare through the whole list every time, and if the usage actually relies on uniqueness then it will possibly fail.
So it probably should be applied in any case if somebody made a pull request.