I'd say it;s pretty much a given that Volta>Pascal will be less than Pascal>Maxwell, least of all because were not gonna go from 16nm to 7nm as was the case from Maxwell to Pascal i.e. 28nm to 16nm. There's also talk that consumer Volta might be more compute heavy, therefore expecting huge gains over current gaming flagships is being overly optimistic.
Change from 192Cores/256 KB Register File Size to 128 Cores/256 KB RFS brought 33% IPC increase, and was reflected in both: gaming and compute benchmarks.
IPC increase from going from 128cores/256 KB RFS, to 64 Cores/256 KB RFS is 50%. Volta may be biggest jump in IPC we have seen in very long time.
In essence: 1024 CUDA core GPU using Volta arch. clocked at 1.5 GHz will have the same level of performance as 1536 CUDA core, 1.5 GHz Pascal arch. GPU.
Also bare in mind that with each generation Nvidia slightly increases core counts in their GPUs.
Kepler to Maxwell GX104 - 1536 CC -> 2048 CC.
Even if Pascal used the same arch as Maxwell in consumer space, we still got massive increase in performance thanks to increased core clocks and core counts:
Maxwell - Pascal GX104 - 2048 -> 2560 CC's.
The same pattern was everywhere.
Thats why I expect that GV104 resembling Volta architecture will have 3072 CUDA cores, with 50% higher IPC, resulting in around 65-70% higher performance than GP104 had.
GV107 - 1024 CC's, with 50% higher IPC, resulting in around 15% higher performance than GTX 1060.
Nvidia this way will be able to say to everybody: Our competition democratized VR. We are democratizing 1440p, and 4K gaming with GTX 2050 Ti, and GTX 2060.