There is no benefit to NVIDIA holding back on perf/W improvements:

* It would make all the cards more expensive with better cooling and better VRMs need.

* It would lock them out of much needed laptop wins where they face real pressure to prove they're even needed, as well as Intel with Iris Pro.

* It would harm them in their GPGPU ambitions that is very perf/W sensitive.

There is no benefit to them holding back on die area/perf because that just directly hurts them in their own margins and makes chip fabrication more risky.

The only performance I could see them leaving on the table is to make a bigger chip, and in the previous generation they covered that with the move from the GTX 980 to the GTX 980 Ti. And that was 9 months later (not 5 years) to put out pretty much the biggest 28nm die they could.