An fx-8350 (which is moderately better then your 8150) needs a bit of overclock to not bottleneck a 970 so yes the 980 would be a noticeable drop.
With watercooling and a good motherboard/psu you should be able to overclock an 8350 high enough to have little or no bottleneck. At that point you might compare the cost to an i5 + new motherboard though.