It varies during a mission and depends on a lot of factors: map, objects, AI, settings... so get a tool that shows you CPU/GPU times (even DCS fps counter kinda shows this) and see for yourself. No matter what HW you have a few settings or different mission can make it either CPU intesive or GPU. If anything, Vulkan will help in optimisation and you won't have to upgrade then, but until this time comes you'll probably upgrade anyway. It all depends on your own requirements - does it play and look nice atm?