i have a 2500k which is virtually the same as your 2400 except im overclocked to 4.5ghz compared to your 3.1ghz. otherwise i have a 980ti and i have absolutely no meaningful bottlnecks and i hit all the same benchmark scores as others do with newer cpus.
i would say a 390/970 would be an excellent upgrade and you wont have any bottlenecks to worry about.
and if you are on 1080p, the "WoW favoring nvidia" isnt going to be relevant with the power of these class gpus as it wont matter that the 390 gives 105fps while the 970 gives 135fps. get whatever you feel is best as they are both excellent cards. though i would personally favor certain 390 models over the 970.