Two threads per core is not about hyperthreading. It's about I/O.

When a render thread needs data from the disk, it is idle and doing nothing. When you put two threads to work sharing a core, then when one is waiting for stuff from disk and the other isn't, the other makes progress. One thread per core usually cannot make progress 100% of the time. Two threads are more likely to do so. If both have work to do and are not waiting for I/O, then they each run half speed, but 2 times 1/2 = 1 anyway.

I worked for a company that did massive multi-processing, with all sorts of combinations of cores, hyperthreads, and multiple processors. The general rule was pretty consistent - optimal overall performance is 2 threads per executable core.

In a 4-core machine with hyper-threading, that's like 8 cores, so you should use 16 threads.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)