Indeed, I did not read the existing specs, assuming it was a new build...
7700K/2 way 1080s?? If playing at 1440P or above, I'm going on record for predicting a 'massive' (i.e., measly!) 1-2 fps increase with an 8700K....
I'd sure want to wait and see how much /if any of a boost is seen in assorted resolution/refresh rate scenarios with your favorite games before making such a purchase. (At least a few folks 'upgraded' to R7-1700 from a 4790K only to be met with frame rate decreases, somewhat disheartening, of course....)