[thesz]

  > Assume FP64 units are ~2-4x bigger.

This is wrong assumption. FP64 usually uses the same circuitry as two FP32, adding not that much ((de)normalization, mostly).

From the top of my head, overhead is around 10% or so.

  > Why would gamers want to pay for any features they don't use?

https://www.youtube.com/watch?v=lEBQveBCtKY

Apparently FP80, which is even wider than FP64, is beneficial for pathfinding algorithms in games.

Pathfinding for hundredths of units is a task worth putting on GPU.

[kbolino]

Has FP80 ever existed anywhere other than x87?

[Paul_Clayton]

The Motorola 88k and 68k both supported (eventually) extended precision, and, of course, Itanium supported it for x87 compatibility.

https://en.wikipedia.org/wiki/Motorola_88000 (under "Registers": "32 80-bit (88110 only)")

https://en.wikipedia.org/wiki/Extended_precision (see section titled "IEEE 754 extended-precision formats")