The bolded is actually not true. The 512MB block is of course faster than system RAM, but the game engine treats system RAM and GPU RAM differently it won't treat the last memory block differently. This is exactly why the 970 stutters and pauses in situations where the 980 does not.
I dont believe that at all. Sure the game engine may differentiate between system ram and vram, but on driver side of things, the GPU should have plenty of room to handle memory usage regardless of what the game dictates to it. In some titles that go above 3.5gb it was very hard to push the 970 past that point (unless you use silly settings that may cripple it anyway regardless of vram). Which yes, may have been a sign that Nvidia did indeed anticipate some problem there. But that is not yet "exactly why the 970 stutters and pauses in situations where the 980 does not".
With some titles (ie Mordor, ACU) where it was difficult to pass 3.5gb, a later driver (347.52)
seems to have slightly upped that limit and cured the stutter where it can pass it more easily with less of a problem. So seems plenty of room for refinements on driver side of things. Bottom line, no one knows for certain to what extent the 970's stuttering performance was from its 3.5 + .5gb design or the card running out of GPU power when pushed in conjunction with poor driver optimization for its unique design.
Still looking for an update to this. It remains a juicy story that should entice many sites or individuals to experiment further... so where are new benches, FCATs, new titles tested since?
As the 780 had no problem with its 3gb pushed to the limits in Mordor, I dont think the 970 will either with its 3.5gb. It may have been a temporary problem early on that was largely resolved.. unless someone brings fresh evidence to the contrary.