At least when not streaming, a graphics card upgrade would undoubtedly provide the most performance benefit when running games at 1440p with high graphics settings. A GTX 980 performs roughly similar to a 1060 6GB, which doesn't exactly have enough performance to comfortably run most recent games well at 1440p without lowering settings a fair amount. It's more of a 1080p card at this point. A 2070, by comparison, offers roughly double the graphics performance at 1440p, making it a lot more suitable for that resolution, and you would likely see a big uplift in performance in most titles.
Most games will still get along fine enough on a quad-core, so a processor upgrade would not improve performance much at all in those cases, as the per-core performance of processors hasn't increased much in recent years. This would especially be true in configurations where the graphics card is what's limiting performance, which would often be the case with a GTX 980 at 1440p. There are a handful of games like Battlefield V that can show some notable performance benefits from having access to more cores, but for the most part developers still try to keep their games running well enough on a quad-core, since that's still what most people have.
As for streaming, that can certainly increase load on the CPU, and will impact performance more substantially on a quad-core processor. The exact performance hit could vary a lot based on the setup though. A CPU-based video encoder would definitely need access to more cores, but a GPU based encoder like NVENC would be much less demanding, and the improved NVENC in the 20-series graphics cards could provide better image quality for the stream over the version in Nvidia's prior generation cards.
In general, I think you would likely get the most performance benefit out of a graphics card upgrade. Your CPU will likely hold back performance when streaming, but your existing graphics card will be holding back performance whether streaming or not with most newer games at 1440p. You may want a CPU upgrade eventually, but unless you are fine with running games at low settings, a graphics card would likely provide the most performance uplift, and would probably be what I would upgrade first.