I got it from that VickyWiki bit you quoted. I interpreted it to mean that the only way to get a factory working at 100% efficiency (aside from other modifiers like Railroads and Industry tech) was to make sure all the POPs working that factory were 50k or higher.
Apparently that's a misunderstanding on my part, however.
So, to further clarify, does this mean that there's a baseline factory efficiency, and POPs just add a bit extra? Like, a factory may have a baseline efficiency of 40%, and filling it with 5 POPs of 1k each would bring the actual efficiency up to 42.5%?
If not, how does efficiency work?