What I have found especially interesting is the story about the undocumented instructions of 8085, because I was not aware of them.

Those instructions would have been actually quite useful and if they had been documented they would have made the Intel 8085 significantly more competitive with Zilog Z80, taking into account also the fact that in the early years 8085 usually had a higher clock frequency (3 MHz or 5 MHz for 8085 versus 2.5 MHz or 4 MHz for Z80).

When I was young I have worked to make some improvements in speed to the functions that implemented the floating-point arithmetic operations in the run-time library used by the Microsoft CP/M Fortran compiler, because they were too slow for my needs (obviously after disassembling them, as Microsoft did not document them). On an Intel 8080 CPU, more than 100 FP64 multiply-add operations per second was considered as high speed, while now a CPU that does 100 billion FP64 multiply-add operations per second is considered a very slow CPU (the best desktop CPUs are more than 6 times faster).

I am sure that with the extra 16-bit operations provided by 8085, a decent speed-up of those FP arithmetic functions would have been possible and I would have found that useful at that time, because I was able to use IBM PC clones only some years later.

The tiny machines at the Datapoint 2200 and 80xx level didn't do anything like that.

At the other extreme, there were low-end machines where the registers really were in main memory. The compute/memory speed ratio has changed over time. Today, arithmetic is much faster than memory, but in the late 1960s/early 1970s, arithmetic was often slower than memory on low-end machines.