Tom makes a good point about films running out of steam much above ISO 1000. There seems to be a ceiling that silver halides won't penetrate. Kodak Royal-X Pan Recording, 2475 Recording, Ilford HPS, Agfa Isopan Record all had a base speed under 2000 and gave best results around 1000. Supposedly the same thing goes with Fuji 1600.
The various "recording" films are just that: designed to record an "image" rather than to produce a pleasant artistic portrayal of the subject's tonality. We can increase development to bring up the highlights, and to a lesser extent the mid-tones, but the shadows below a certain point, they just ain't there!
What has improved in the years since Royal-X Pan Recording first came out the the early sixties is grain. We used to refer to it as "golf ball grain" Now we have films that are as fine grained as Tri-X was back then.
Victor's Mercury Intensifier had a warning label about mercury being poisonous, but this was an era when during science class the fourth grade teacher would let the children roll a little ball of mercury around on the palm of their hands to demonstrate that a metal could exist in liquid form at room temperature. We were just told not to eat the stuff. You can get a similar speed boost by toning your negatives with selenium toner.
But back to film speed. If you want some semblance of shadow detail, avoid excess contrast, and unblocked highlights there are a number of developers and developing regimins that can get you a USEABLE negative, but not a great one, at perhaps ISO 3200. There were many times when an image that would be recognizeable in the newspaper was all the editor wanted. Nobody was looking for great art, and the 65 lines per inch halftone screen and letterpress printing on newsprint would pretty much obscure the film grain anyway.
So to sum it up, back about 1962 we finally got a film, Royal-X Pan recording, with a suggested speed of about ISO 1250, and now forty seven years later Fuji has trounced Kodak's record by 1/3 of a stop at ISO 1600. Maybe! We'll never really know because we can't compare them with one another directly, same meter, same lens, same lighting, same subject.
I just walked down my hallway and looked at two framed prints. One is from 1962, shot on Royal-X Pan Recording, the other was shot on 2475 in 1968, both developed in Acufine and exposed at 3200. There's no way to duplicate that look today, and that's the shame of "progress".