The results you get with a film like TMZ are affected by many factors. You can always get an increased contrast index by extending developing time. If you extend it too long you will get acceptable highlights but still poor shadow detail. If the light is low enough that you must expose TMZ at a higher speed than 800, that's what you have to do. The developer you use makes an important difference. A developer like Microphen, which is based on phenidone, will give you a little more speed than a developer like D-76, which is based on metol. Exposure is, of course, also very important. Someone who shoots Tri-X and insists it should be rated at 200 rather than 400 may be doing some of these things: using a camera with a meter which is off by one stop, metering improperly off of a very light colored subject, using a thermometer which is slightly off, underdeveloping. If you are using undiluted Microdol-X or Perceptol then you would need to rate Tri-X at 200. In most other developers, 400 wold be fine. By far the nost common problem with "rating" a film is underexposure when metering off of a subject which is lighter than 18% gray. Using a spot meter will not solve the problem. You have to understand what you are metering off of. In light which is not too high or low in contrast if you meter off of a white subject, you need to add two stops of exposure. This doesn't mean you are rating TriX at 100. It means that if you were metering off of an 18% gray card, it would show two stops of extra exposure compared with the reading taken from the white subject.
Roll film shooting is not like digital shooting. You can't simply dial up the ISO for various shots and not for others. That's why a faster film can be useful. Fuji's Neopan 1600 is already out of production. TMZ may or may not still be in production. I do not have a problem getting 800 from TMZ. It must be handled very carefully, especially during loading, to avoid fogging but it can be done.