In your shoes, which I kind of was, I would wait until you can upgrade the whole system. I went from a i7 3930k & an Asus X79 Rampage IV Extreme to the setup in my signature, that 1080 Ti is much happier in the new system especially in CPU-intensive games. Spending $300 on a dead-end upgrade for a CPU on that setup just seems like a complete waste of money, to me anyway. You'd be better off saving up for however long it takes & getting an LGA 1151 or Ryzen 2nd/3rd gen (if they're out by the time you buy) setup.
Just my two cents.