FlashCode suggested a nice solution to this problem with nicks (and other case-insensitive items):
Suppose there are nicks "Flash" and "flashy" in a channel. When trying to address "Flash" one types "Fla" and uses partial completion. Current behaviour completes it to "flash".
FlashCode suggested that WeeChat should check if partial completion matches an item case-insensitively (in this example partial completion of "Fla" is "flash" and it matches an item "Flash) and if it is so, it should capitalise the completion just like the item is capitalised.
It works well with case-insensitive items, but the problem of paths and other case-sensitive items is still open.