My car is ten years old. Is it worth it to upgrade to high performance struts and springs?

Why not? If you like the car, maintain it. Cheaper than a new one, and if you can't afford to upgrade something this month the bank won't come and get it. If it was a suzuki tracker or another piece of crap I would say no, but your truck is like my saab, well built, fun to drive and if some day I have to drop a new motor and in it, so be it, 4 grand is a lot better than 25k