[hinkley]

Could you, do they, put the “extra” LUs right next to the parts of the chip with the highest average thermal dissipation to even out the thermal load across the chip?

Or stack them vertically, so the least consistently used parts of the chip are farthest away from the heat sink, delaying throttling.

[bcrl]

Intel has internal papers that investigated the use of the third dimension and the effect it would have on power consumption and performance. Of course it improves things, but it is very difficult to implement in the real world. The first real use of this technique by Intel is coming soon in the form of backside power delivery.

AMD 3D-Vcache technology shows that stacking an additional layer of transistors has a significant effect on thermal limits of a modern CPU. The extra cache is strategically placed over parts of the CPU die that use less power, yet those CPUs still have to run at lower temperatures and power settings compared to their non-vcache models. Just because you can build it doesn't mean that it will be a good fit for the mass market.