CUDA only helps the likes of premiere, FCPX uses OpenCL, you didn't mention which editing program you use, but I am guessing FCPX?
BTW, I use two monitors, with a R9 280x, and FCPX, does a great job BTW. But one thing, don't upgrade to 10.11 as I have a few bugs with 10.11 and FCPX at the...