Of course the latitude is really only to overexposure. Once something is off the bottom it is gone, though it may not be an issue for a particular picture that doesn't have important shadow detail that far down.
I do understand Kodak's pushing recommendation and it makes sense since increasing the development time doesn't recover the lost shadow detail.
This morning I extended the test Bill linked to above (indirectly, same dog but different lighting/scene) with a shot at the tail of another roll.
That shot proved for me, for Delta 400 in DD-X at least, the point you make in your last sentence. I got nice detail where I wanted it from a frame incident metered and shot at 1600 but developed for EI 500. I'm going to try a frame or two shot at 3200 and developed at 500 on the next roll.
The camera meter though, center weighted and matrix, in my F5 was being seriously fooled by the white background telling me I'd be 2-stops over exposed, had I followed the camera's lead the shot would have lost the detail I wanted. (Good spot metering and zoning techniques would have worked fine too, they would simply have taken a bit more thought.)
Well, the straight line assumes that ALL is within that straight line. I believe that at box speed that is on the cusp and that any further reduction in exposure BEGINS (not necessarily perceptive to most) to reduce the shadow detail's presence on that straight line. I would begin to give ever so slightly more development with an EI 200 for TMX.
David I'll definately grant that the application of these principles needs to fit the individual user and that as exposure is reduced there is less and less latitude at the shadow end of the scale.
One point I'm trying to make though is that normally I don't print clear to the toe. I don't think I'm odd in this respect either. Yes, a certain number of us shoot tight enough to the toe or long enough scale subjects that a small change in EI might really matter. It is obvious from my tests that I'm not in that group.
A second point that I want to make, and that Mark Crabtree also made above, is basically summed up in the old axiom "expose for the shadows and develop for the highlights"; development changes have very little effect on film sensitivity and shadow detail; it's almost all about the exposure when we are placing subject matter down close to the toe. Changing your development from 100 style to 200 style is about fitting the scene to the paper not getting more shadow detail.
I think it is more likely that the very different highlight contrast of the two films is the reason. I shoot a fair bit in full sun and in available indoor light. In those sorts of high contrast situations I think the very slight tapering of the highlights with TMX compared to the straight line (or sometimes slightly upswept) highlight curve with TMY means you can give more development with TMX (or need to give less with TMY, depending on how you want to look at it).
my results, your may be different