Concrete benefits in the brain from synesthesia is an interesting topic. The article says it has given people a different source of creativity but not much else. It’s difficult to say if synesthesia has improved my brain. To me it feels mostly seperate from my cognitive skills.
I noticed that when I visualize songs as written lyrics particularly in foreign languages, my ability to hear the precise lyrics which I would normally miss 10/10 times is drastically enhanced.
So I think there are definitely even elementary benefits to synesthesia.