5.5B to 6B transistors
I had to look this up, and sure enough, GM107 is 1.87B transistors. *3 of that is 5.61B. Now whether that kind of simple scaling is really realistic I don't know. But it does look like they had to come up with something new when Maxwell 20 had to be canned, so doing something that doesn't stray much from a linear increase from GM107 would be the easiest way to get something out.
A 1920 core 880 would still be faster than the 780 Ti though.. at lower res at least.