This new iMac pro, in its ‘low end’ configuration, has 8 cores (https://www.apple.com/lae/imac-pro/specs/), possibly of the new “Purley” kind (http://bgr.com/2017/07/12/imac-pro-2017-specs-intel-xeon-pur...)

A single A10 will not add 50% of computing power, even ignoring its GPU.

True, but the A10 is much cheaper than any Intel CPU, so Apple could develop a multi-socket ARM motherboard to increase the number of ARM cores, to make performance compatible with Intel's (perhaps at lower cost, since Apple can get cheap A11s using the high volume of iPhone 8/X).

Also, according to Geekbench, the single-thread performance of the 2.4 GHz A11 is 70%[1] of the 3.7 GHz i7-8700K. I'm guessing Apple could bring the A11 even closer to the i7 by increasing the clock frequency, which may not make sense for mobile devices (because of disproportionally higher power consumption).

So, again according to the Geekbench figures, four A11s (at mobile clock frequency) is 50% faster than a 6-core i7-8700K[1].

[1] https://browser.geekbench.com/ios_devices/52 https://browser.geekbench.com/processors/2062

If the A10 really is half as fast as an Intel CPU, wouldn't it make more sense for Apple to put four of these chips in an iMac -- delivering double the performance -- and use the savings to fight Intel's x86 emulation patent in court?

One could hope that, due to the negative PR, Intel would give up trying to prevent Apple from delivering 2x the performance at a lower cost. In any case, I think this patent needs to be challenged thoroughly, as it seems to favor only Intel while punishing consumers.