I didn't believe this at first, but I just decided to try and test it myself with texture modded Skyrim and my SLI 970s. I tried to push the 3.5 GBs barrier by downsampling it from 5120x2880 with the four following experimental conditions:
1. No MSAA applied on top
2. 2xMSAA applied on top
3. 4xMSAA applied on top
4. 8xMSAA applied on top
Since MSAA is known to be VRAM heavy, it made sense. I also kept a close eye on GPU usage and FPS with the Rivatuner overlay as well as VRAM usage. All of this was done running around Whiterun to minimize GPU usage. My results were as follows.
1. Skyrim peaked at about 3600 MBs in usage with occasional brief hitching while loading new textures in and out of VRAM. GPU usage remained well below 99% on each card.
2. Skyrim once again peaked at about 3600 MBs with the mentioned hitching, this time somewhat more frequently. Once again, GPU usage remained well below 99%.
3. Skyrim yet again peaked at about 3600 MBs and hitched much more prominently and frequently at the same time as VRAM usage droppped down 100-200 MBs. GPU usage was below 99% again with FPS still at 60 aside from those hitches.
4. Now Skyrim was using the full 4 GB framebuffer with massive stuttering and hitching from a lack of VRAM. This time, I had to stare at the ground to keep GPU usage below 99% and retain 60 FPS. I ran around Whiterun just staring at the ground and it remained at 60 FPS except with those massive hitches where GPU usage and framerate temporarily plummeted. This last run merely indicated that Skyrim can indeed use more VRAM than it was with the previous 3 settings and so the issue seems to be with the 970s themselves rather than just the game in this example. The performance degradation aside from VRAM was severe, but that could just be 8xMSAA at 5K taking its calculative toll.
So it seems to me that my 970s refuse to utilize above ~3600 MBs of VRAM unless they absolutely need it, but I've no idea why. Nvidia didn't gimp the memory bus in any overly obvious way from the full GM204 chip therefore the 970s should have no issue using the same VRAM amount as the 980s. I don't like what I see, it's like the situation with the GTX 660 that had 2 GBs but could only effectively use up 1.5 without reducing its bandwidth to a third, so it tried to avoid exceeding 1.5. The difference is that was predictable due to the GK106's 192-bit memory bus, there's nothing about the 970's explicit specifications that indicates the same situation should apply.
A similar shortcoming was noticed sometime back regarding the 970's ROPs and how the cutting-down of 3 of GM204's 16 SMM units affected the effective pixel fillrate of the 970s despite retaining the full 64 ROPs. It's possible that Maxwell is more tightly-connected to shader clusters and severing them affects a lot about how the chip behaves, but that doesn't really make sense. If this is an issue, it's almost certainly software-related. I'm not happy regardless of the reason and I'll try more games later. Anecdotally, I have noticed recent demanding games peaking at about 3500-3600 MBs and can't actually recall anything going beyond that. I didn't pay attention to it or change any conditions to test it.
