Not sure how this thread degenerated into an offtopic argument about Mantle but my 2 cents is that if you want to maximise bang for the buck, an aftermarket 290 is the way to go. If you're not allergic to buying a used card, this is even more so.
As for the GTX 980. There is a higher delta in 1080p than in 1440p, as for all Maxwell cards. Don't know why people bring up 1440p all the time, since it is a very, very marginal resolution(per the Steam survey). I guess it helps AMD more in benchmarks.
From TPU's GTA graph for 1080, the GTX 980 is about 12.4% faster than the 390X at stock speeds - and that is before overclocking, which is much better on the 980.
Still, I agree generally speaking that the 980/390X are both overkills. The 290 is imho still an amazing card giving amazing value if you can find it for sub-250 dollars.