Well I can tell you from experience the 1080GTX is 25% faster than 770 Sli. That's what I replaced when I slapped in my 1080 GTX F|E from Evga. I noticed a large performance increase over my 770 SC Sli.
Using a i5 4690k @ 4.7GHZ on a Asus Sabertooth Mark 2 Z97 board-
Valley 1.0 EVGA 770 SLI =95.6 fps
EVGA 1080GTX F|E= 109-113 fps
Heaven 4.0 EVGA 770 SC SLI- 109-111 fps
EVGA 1080 GTX F|E= 135-137 fps
Firestrike 1080pEVGA 770 SLI= 12,170 highest bench

EVGA 1080 GTX F|E 16,717
Huge difference between
770 SLI and a 1080 GTX!
You mentioned you were just using 1x 770 GTX? that's pretty crazy to say that a 8GB Vram card with GDDR5X is even comparable to a single 2GB VRam 770 GTX!